Pressure-sensitive adhesive sheet

ABSTRACT

Provided is a PSA sheet that can be used on a metal adherend because it has anti-metal-corrosive properties and can also maintain good removability while preventing or inhibiting adherend surface contamination even when prolongedly adhered to the adherend. A PSA sheet having a PSA layer is provided. The PSA layer comprises at least 95% base polymer by weight and has a chloride ion amount of less than 300 μg per gram of pressure-sensitive adhesive layer, determined by hot water extraction.

TECHNICAL FIELD

The present invention relates to a pressure-sensitive adhesive sheet.

The present application claims priority to Japanese Patent Applications No. 2018-246231, No. 2018-246232 each filed on Dec. 27, 2018 and No. 2019-118312 filed on Jun. 26, 2019; the entire content thereof is incorporated herein by reference.

BACKGROUND ART

In general, pressure-sensitive adhesive (PSA) exists as a soft solid (a viscoelastic material) in a room temperature range and has a property to adhere easily to an adherend with some pressure applied. For such a property, in processing and transporting various articles, PSA has been widely used as PSA sheets for various purposes, including surface protective sheets to prevent damage (scratches, contamination, corrosion, etc.) of their surfaces. For instance, a PSA sheet having a PSA layer on one face of a substrate is preferably used as a surface protective sheet to protect metal plate surface. Conventional art documents disclosing this type of known technology include Patent Document 1.

CITATION LIST Patent Literature [Patent Document 1] Japanese Patent Application Publication No. 2017-186517 SUMMARY OF INVENTION Solution to Problem

For instance, PSA sheets used as protective sheets are applied to adherends for temporal periods including the periods requiring their protection (e.g. while being processed, transported, etc.). To protect adherend surface while in use, a PSA sheet used in such an embodiment needs to have an ability to well adhere to the adherend (adhesive strength). For instance, for a purpose to protect metal plates, an ability to prevent their corrosion is required.

After serving the protection purpose, the PSA sheet for protection is removed from the adherend. However, when used in an embodiment involving prolonged protection by the PSA sheet, the adhesive strength may increase due to changes in temperature, etc., and the intended removability may not be maintained. As a solution, an adhesive strength adjuster may be added to reduce the increase in aged adhesive strength; however, the adhesive strength adjuster may contaminate the adherend surface.

The present invention has been created in view of such a problem with an objective to provide a PSA sheet that can be used on a metal adherend because of its anti-metal-corrosive properties and can also maintain good removability while preventing or inhibiting contamination on the adherend surface even when adhered to the adherend for a long period.

Solution to Problem

The present description provides a PSA sheet having a PSA layer. The PSA layer comprises at least 95% base polymer by weight. It has a chloride ion amount of less than 300 μg per gram of PSA layer, determined by hot water extraction.

By using the PSA comprising at least 95% base polymer by weight, the PSA sheet can maintain good removability while preventing or inhibiting contamination of adherend surface even when adhered to the adherend for a long period.

When the PSA's chloride ion amount is below the prescribed value, anti-metal-corrosive properties can be obtained without adding an anti-corrosive component such as rust inhibitor or while limiting its usage. Thus, the PSA sheet can also be preferably used in an embodiment applied to metal. With respect to this, the present inventors have found that corrosion of the metal subject to application can be caused not only by external factors such as contacts with water, but also by the PSA sheet itself, and have learned that the corrosion is caused by chloride ions in the PSA sheet. The anti-metal-corrosion effects of the art disclosed herein is based on this knowledge.

In summary, according to the embodiment, even when adhered to a metal adherend for a long period, it can maintain good removability while preventing or inhibiting adherend surface contamination and can also prevent metal corrosion in the adherend.

In some preferable embodiments, the PSA layer is formed from a water dispersed PSA composition. From the standpoint of the environmental sanity, etc., water dispersed PSA compositions are more desirable than solvent-based PSA compositions in which adhesive components are dissolved in organic solvents. In such an embodiment, the effect (combination of aged removability and anti-corrosive properties) of the art disclosed herein can be preferably obtained.

In some preferable embodiments, the FSA layer includes 3% rust inhibitor by weight or less. The PSA layer in this embodiment is free of a rust inhibitor or includes up to the prescribed level of rust inhibitor. While rust inhibitor is an effective additive for preventing corrosion, an increase in its usage tends to result in compromised aged removability. By limiting the use of rust inhibitor to or below the prescribed level, the increase in aged adhesive strength can be reduced to preferably maintain good removability. It is also significant to limit the rust inhibitor usage in view of highly inhibiting contamination of adherend surface. For instance, when the adherend surface is subjected to a process such as coating after removal of the PSA sheet adhered for a certain time period, defects (coating defects, etc.) due to adherend contamination can be avoided in the process.

In some preferable embodiments, the base polymer is selected among acrylic polymers and rubber-based polymers. Preferable PSA (layers) in the art disclosed herein include acrylic PSA comprising an acrylic polymer and rubber-based PSA comprising a rubber-based polymer.

In some preferable embodiments, the base polymer is crosslinked with a crosslinking agent. By using a polymer crosslinked with a crosslinking agent as the base polymer, a PSA sheet having good removability is preferably obtained. Favorable examples of the crosslinking agent include oxazoline-based crosslinking agents. In an embodiment using an oxazoline-based crosslinking agent, the effect (aged removability) of the art disclosed herein can be preferably obtained.

The PSA sheet according to some preferable embodiments has a substrate layer. The substrate layer supports a PSA layer and the PSA sheet has a substrate layer and a PSA layer formed on one face of the substrate layer. This embodiment is suited for surface protection of metal plates, etc. The substrate layer is more preferably formed from a resin film. As such a substrate layer, it is preferable to use a substrate layer formed from a polyolefinic resin film, a substrate layer formed from a polyester-based resin film, or a substrate layer formed from a vinyl chloride resin film.

The PSA sheet disclosed herein preferably has a 180° peel strength (to-metal-plate aged adhesive strength) of 2.7 N/20 mm or less, relative to a metal plate selected between a stainless-steel plate and an aluminum plate, after applied to the metal plate and stored at 50° C. for one week. The PSA sheet satisfying this property can be removed well from the adherend as the aged adhesive strength is sufficiently suppressed even when adhered to the adherend for a long period. It is noted that as the to-metal-plate aged adhesive strength, the aged to-SUS adhesive strength and/or the aged to-aluminum adhesive strength described later is used.

In an application adhered to a metal plate, even when adhered to the metal plate for a long period, the PSA sheet disclosed herein can maintain good removability while preventing or inhibiting contamination of adherend surface and can also prevent corrosion of the metal plate. Thus, it is preferably used for protecting metal plates. After the protection period by the PSA sheet ends, the metal plate (e.g. an aluminum plate) can be subjected to surface coating, etc. Accordingly, it is desirable that adherend contamination is highly inhibited as it may badly affect the coating, etc. By applying the art disclosed herein for metal plate protection, after the protection by the PSA sheet ends, the metal plate may still be free of defects (coating defects, etc.) even in post-processes such as coating. The PSA sheet disclosed herein is particularly preferably used for protecting aluminum plates.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a cross-sectional diagram schematically illustrating an embodiment of the PSA sheet.

DESCRIPTION OF EMBODIMENTS

Preferred embodiments of the present invention are described below. Matters necessary to practice this invention other than those specifically referred to in this description can be understood by a person skilled in the art based on the disclosure about implementing the invention in this description and common technical knowledge at the time of application. The present invention can be practiced based on the contents disclosed in this description and common technical knowledge in the subject field.

As used herein, the term “PSA” refers to, as described earlier, a material that exists as a soft solid (a viscoelastic material) in a room temperature range and has a property to adhere easily to an adherend with some pressure applied. As defined in C. A. Dahlquist, “Adhesion: Fundamental and Practice” (McLaren & Sons (1966), P. 143), the PSA referred to herein is a material that has a property satisfying complex tensile modulus E*(1 Hz)<10⁷ dyne/cm² (typically, a material that exhibits the described characteristics at 25° C.).

The concept of PSA sheet herein may encompass so-called PSA tape, PSA labels, PSA film, etc. The PSA sheet disclosed herein can be in a roll or in a flat sheet. Alternatively, the PSA sheet may be processed into various shapes.

Constitution of PSA Sheet

The PSA sheet disclosed herein typically has a PSA layer on a substrate layer (support substrate). FIG. 1 shows a cross-sectional structure of the PSA sheet according to an embodiment. PSA sheet 10 comprises a PSA layer 2 provided on the first face 1A of a substrate-layer sheet 1. When used, it is applied to an adherend over the face 2A of PSA layer 2. When PSA sheet 10 is used as a surface PSA sheet, the face 2A of PSA layer 2 is applied to an object to be protected. The back face 1B (on the reverse side of the first face 1A) of substrate layer 1 is also the back face of PSA sheet 10, forming the outer surface of PSA sheet 10. Prior to use (i.e. before applied to the adherend), PSA sheet 10 can be in a form where the face 2A (adhesive face, i.e. the bonding surface to the adherend) of PSA layer 2 is protected with a release liner (not shown in the drawing) having a release face at least on the PSA layer side. Alternatively, PSA sheet 10 may be in a form where, with the second surface (back face) 1B of substrate layer 1 being a release face, PSA sheet 10 is wound in a roll so that the back face comes in contact with the PSA layer 2 to protect the surface (adhesive face) 2A.

The PSA sheet can be a substrate-bearing double-faced PSA sheet in an embodiment having a PSA layer on each face of a substrate layer, or a substrate-free PSA sheet in an embodiment where the PSA layer is supported on a release liner (which can be thought as a substrate having a release face) or like embodiment. In this case, the PSA sheet can be formed solely of the PSA layer.

As the release liner, commonly-used release paper and the like can be used without particular limitations. For instance, a release liner having a release layer on a surface of a liner substrate such as plastic film and paper, a release liner formed from a low-adhesive material such as fluorinated polymer (polytetrafluoroethylene, etc.) and polyolefinic resin, and the like can be used. The release layer can be formed by subjecting the liner substrate to surface treatment with various release agents including silicone-based, long-chain alkyl-based, and fluorinated kinds as well as molybdenum sulfide.

The PSA sheet disclosed herein is not particularly limited in width. For instance, by using a PSA sheet having a width of about 0.5 m or greater or even about 1.0 m or greater, an adherend (aluminum plate, stainless-steel plate, etc.) having a relatively large surface area can be efficiently covered. In some preferable embodiments, the PSA sheet has a width of about 1.5 m or greater. With such a broad PSA sheet, an adherend with a large surface area can be covered with one sheet or a few sheets. The maximum width of the PSA sheet is not particularly limited. From the standpoint of the productivity, handling properties, etc., it is suitably about 5 m or less, for instance, possibly about 4 m or less.

The PSA sheet disclosed herein can be specified by the plane (sheet face) having a long side and a short side. The long side is defined longer than the short side and the short side is defined shorter than the long side. For instance, the short side may orthogonally intersect the long side. The PSA sheet's length direction coincides with the long side and the width direction orthogonally intersects the length direction. Accordingly, as used herein, the “width” is defined as the length (distance) in the direction orthogonal to the length direction. Long-length, belt-like and oblong PSA sheets are typical examples of the PSA sheet disclosed herein. While the long side extends almost linearly, the short side is not limited to a straight line and can be a curve, zigzag, etc. For instance, in a long PSA sheet, the length (distance in the length direction) is equal to or greater than the width.

The thickness of the PSA sheet disclosed herein is not particularly limited. From the standpoint of the handling properties, the lightness of weight, etc., it is suitably about 1000 μm or less (typically about 300 μm or less, e.g. about 150 μm or less). In some embodiments, the thickness of the PSA sheet is preferably about 120 μm or less, more preferably about 100 μm or less, yet more preferably about 75 μm or less, or possibly, for instance, less than 60 μm. The thickness of the PSA sheet can be typically greater than 20 μm, preferably greater than 30 μm, or more preferably greater than 40 μm, for instance, greater than 45 μm.

As used herein, the thickness of the PSA sheet includes the thicknesses of the PSA layer and the substrate layer, but excludes the thickness of the release liner.

Properties of PSA Sheet Chloride Ion Amount

The PSA sheet disclosed herein is characterized by having a chloride ion amount of less than 30 μg per gram of PSA sheet, determined by hot water extraction. Even in an embodiment applied to metal, the PSA sheet with such a limited chloride ion amount helps obtain a configuration that brings about long-term prevention of metal corrosion, without adding an anti-corrosive such as rust inhibitor or while limiting its usage. The chloride ion amount per gram of PSA sheet is suitably below 25 μg (e.g. below 20 μg), preferably below 15 μg, more preferably below 12 μg, or yet more preferably below 10 μg. In some preferable embodiments, it has a chloride ion amount of less than 8.9 μg (e.g. about 8.5 μg or less) per gram of PSA sheet, determined by hot water extraction. In some embodiments, the chloride ion amount per gram of PSA sheet is preferably below 8.0 μg, more preferably below 7.0 μg, yet more preferably below 6.5 μg, or particularly preferably below 6.0 μg (e.g. below 5.5 μg). The minimum chloride ion amount per gram of PSA sheet is ideally 0 μg; however, from the standpoint of the productivity, allowable level for practical use, etc., it can be about 1 μg or greater, about 3 μg or greater, or even about 5 μg or greater. In other embodiments, the chloride ion amount per gram of PSA sheet can be about 8 μg or greater, or even about 12 μg or greater. According to the art disclosed herein, by using a limited amount of rust inhibitor in the PSA sheet having such a chloride ion amount, good anti-corrosive properties can be obtained without compromising the aged removability. The chloride ion amount per gram of PSA sheet is determined by the method described later in Examples.

Initial to-SUS Adhesive Strength

The PSA sheet disclosed herein suitably has an initial peel strength (initial to-SUS adhesive strength) of 0.01 N/20 mm or greater, determined at a tensile speed of 0.3 m/min at a peel angle of 180° at 30 minutes after applied to a stainless-steel plate (SUS plate). In some embodiments, the initial peel strength can be about 0.05 N/20 mm or greater (e.g. about 0.1 N/20 mm or greater). In some preferable embodiments, the initial peel strength can be about 0.5 N/20 mm or greater (e.g. about 1 N/20 m or greater), or even about 1.5 N/20 mm or greater. The PSA sheet having the prescribed initial peel strength or greater can bond well to an adherend in relatively short time after applied to the adherend. It may also be less susceptible to the sort of lifting and peeling from the adherend. The maximum initial peel strength is not particularly limited. From the standpoint of light-peel properties, it is suitably about 5 N/20 mm or less, preferably about 2.7 N/20 mm or less, or more preferably about 2.5 N/20 mm or less (e.g. about 2 N/20 mm or less). For instance, even when the adherend is a thin metal plate, the PSA sheet showing such a limited initial peel strength can be smoothly removed from the adherend while inhibiting deformation of the adherend. The initial peel strength is determined by the method described later in Examples.

Initial to-Aluminum Adhesive Strength

The PSA sheet disclosed herein suitably has an initial peel strength (initial to-aluminum adhesive strength) of 0.01 N/20 mm or greater, determined at a tensile speed of 0.3 m/min at a peel angle of 180° at 30 minutes after applied to an aluminum plate. In some embodiments, the initial peel strength can be about 0.05 N/20 mm or greater (e.g. about 0.1 N/20 mm or greater). In some preferable embodiments, the initial peel strength can be about 0.5 N/20 mm or greater (e.g. about 1 N/20 mm or greater), or even about 1.5 N/20 mm or greater. After applied to an adherend typically such as an aluminum plate, the PSA sheet having the prescribed initial peel strength or greater can bond well to the adherend in relatively short time. It may also be less susceptible to the sort of lifting and peeling from the adherend. The maximum initial peel strength is not particularly limited. From the standpoint of light-peel properties, it is suitably about 5 N/20 mm or less, preferably about 2.7 N/20 mm or less, more preferably about 2.5 N/20 mm or less (e.g. about 2 N/20 mm or less), or possibly even about 1.7 N/20 mm or less. For instance, even when the adherend is a thin aluminum plate, the PSA sheet showing such a limited initial peel strength can be smoothly removed from the adherend while inhibiting deformation of the adherend. The initial peel strength is determined by the method described later in Examples.

Aged to-SUS Adhesive Strength

The PSA sheet disclosed herein preferably has an aged peel strength (aged to-SUS adhesive strength) of 2.7 N/20 mm or less, determined at a tensile speed of 0.3 m/min at a peel angle of 180° after applied to a stainless-steel plate (SUS plate) and stored at 50° C. for 7 days. Even when adhered to an adherend for a relatively long period, with sufficiently reduced aged adhesive strength, the PSA sheet satisfying this property can maintain light-peeling properties relative to the adherend. Thus, it shows excellent efficiency for removal from the adherend. According to the PSA sheet having an aged peel strength of about 2.5 N/20 mm or less (more preferably about 2.2 N/20 mm or less, or yet more preferably about 2.0 N/20 mm or less), greater removal efficiency (aged removability) can be obtained. From the standpoint of inhibition of lifting and peeling, the aged peel strength is suitably about 0.05 N/20 mm or greater, preferably about 0.1 N/20 mm or greater, or more preferably about 0.3 N/20 mm or greater. The aged peel strength can also be about 1 N/20 mm or greater (e.g. about 1.5 N/20 mm or greater). The aged peel strength is determined by the method described later in Examples.

Aged to-Aluminum Adhesive Strength

The PSA sheet disclosed herein preferably has an aged peel strength (aged to-aluminum adhesive strength) of 2.7 N/20 mm or less, determined at a tensile speed of 0.3 m/min at a peel angle of 180° after applied to an aluminum plate and stored at 50° C. for 7 days. Even when adhered to an adherend typically such as an aluminum plate for a relatively long period, with sufficiently reduced aged adhesive strength, the PSA sheet satisfying this property can maintain light-peeling properties relative to the adherend. Thus, it shows excellent efficiency for removal from the adherend. According to the PSA sheet having an aged peel strength of about 2.5 N/20 mm or less (more preferably about 2.2 N/20 mm or less, or yet more preferably about 2.0 N/20 mm or less, e.g., about 1.8 N/20 mm or less), greater removal efficiency (aged removability) can be obtained. From the standpoint of inhibition of lifting and peeling, the aged peel strength is suitably about 0.05 N/20 mm or greater, preferably about 0.1 N/20 mm or greater, or more preferably about 0.3 N/20 mm or greater. The aged peel strength can also be about 1 N/20 mm or greater (e.g. about 1.5 N/20 mm or greater). The aged peel strength is determined by the method described later in Examples.

To-SUS Peel Strength Ratio

While no particular limitations are imposed, the PSA sheet disclosed herein may have a ratio of aged peel strength (aged to-SUS adhesive strength) P_(SUS2) (N/20 mm) to initial peel strength (initial to-SUS adhesive strength) P_(SUS1) (i.e. a P_(SUS2)/P_(SUS1) ratio value) of 5 or less. A small P_(SUS2)/P_(SUS1) ratio value means a small increase in peel strength with aging relative to, for instance, a metal adherend. For instance, in an embodiment applied to metal, this favorably combines initial adhesion and light-peeling properties for removal. From such a standpoint, the P_(SUS2)/P_(SUS1) ratio value is preferably 4 or less, more preferably 3 or less, yet more preferably 2 or less, for instance, possibly 1.8 or less, 1.5 or less, 1.3 or less, 1.2 or less, or even 1.1 or less. The P_(SUS2)/P_(SUS1) ratio value is typically 0.8 or greater, for instance, possibly 1 or greater.

To-Aluminum Peel Strength Ratio

While no particular limitations are imposed, the PSA sheet disclosed herein may have a ratio of aged peel strength (aged to-aluminum adhesive strength) P_(AL2) (N/20 mm) to initial peel strength (initial to-aluminum adhesive strength) P_(AL1) (i.e. a P_(AL2)/P_(AL1) ratio value) of 5 or less. A small P_(AL2)/P_(AL1) ratio value means a small increase in peel strength with aging relative to, for instance, an aluminum adherend. For instance, in an embodiment applied to metal, this favorably combines initial adhesion and light-peeling properties for removal. From such a standpoint, the P_(AL2)/P_(AL1) ratio value is preferably 4 or less, more preferably 3 or less, yet more preferably 2 or less, for instance, possibly 1.8 or less, 1.5 or less, 1.3 or less, 1.2 or less, or even 1.1 or less. The P_(AL2)/P_(AL1) ratio value is typically 0.8 or greater, for instance, possibly 1 or greater.

VOC Emission

From the standpoint of environmental sanity, the PSA sheet disclosed herein preferably has a total amount of volatile organic compounds (VOC) of 1000 μg or less emitted from 1 g thereof (possibly including a substrate layer and a PSA layer, but not a release liner) when heated at 80° C. for 30 minutes (or, for short, a VOC emission of 1000 μg/g or less). The VOC emission is more preferably 500 μg or less (e.g. 300 μg/g or less, typically 100 μg/g or less). The VOC emission is determined by the next method.

VOC Test

The PSA sheet is cut to a prescribed size (to an about 5 cm² sized area, here) and the release liner is removed to prepare a test piece. The test piece is placed and sealed in a 20 mL vial. The vial is then heated at 80° C. for 30 minutes and 1.0 mL of the heated gas (sample gas) is injected into a gas chromatograph (GC), using a head space autosampler (HSS). Based on the resulting gas chromatogram, the amount of gas emitted from the test piece is determined as an n-decane equivalent amount. From the resulting value, the VOC emission is determined per gram of PSA sheet (μg/g, excluding the release liner). The n-decane equivalent amount is determined by applying a calibration curve prepared in advance for n-decane, with the intensities of emission gas obtained by GC Mass seen as equivalent to the intensities of n-decane. The HSS and GC settings are as follows:

HSS: model 7694 available from Agilent Technologies

Heating time: 30 min

Pressurization time: 0.12 min

Loop filling time: 0.12 min

Loop equilibration time: 0.05 min

Injection time: 3 min

Sample loop temperature: 160° C.

Transfer line temperature: 200° C.

GC: model 6890 available from Agilent Technologies

Column: J&W capillary column, product name DB-ffAP, available from GL Science (0.533 mm inner diameter×30 m length, 1.0 μm thick membrane)

Column temperature: 250° C. (temperature raised at 10° C./min from 40° C. to 90° C., followed at 20° C./min to 250° C. and maintained there for 5 min)

Colum pressure: 24.3 kPa (constant flow mode)

Carrier gas: helium (5.0 mL/min)

Injection port: split (split ratio 12:1)

Injection port temperature: 250° C.

Detector: FID

Detector temperature: 250° C.

The PSA sheet disclosed herein may have a level of erosion inhibition graded as no discoloration in the aluminum plate corrosion test (at 40° C. and 92% RH for 7 days) carried out by the method described later in Examples. The PSA sheet satisfying this property can exhibit excellent anti-corrosive properties even in an embodiment where the adherend is formed of aluminum; and therefore, with few limitations imposed on the subject of application, it can be used on various adherends.

PSA Layer

The PSA layer disclosed herein is characterized by comprising at least 95% base polymer by weight. This can help limit the usage of additives that affect the aged removability and maintain good removability even when the PSA sheet is prolongedly adhered to an adherend. When the PSA is largely formed of base polymer, the transfer of additives and the like to the adherend surface is limited and contamination of the adherend surface can be prevented or inhibited. For instance, when the adherend surface is subjected to a process such as coating after removal of the PSA sheet adhered for a certain time period, this is significant in view that defects (coating defects, etc.) can be avoided in the process. From the standpoint of the aged removability, the base polymer content of the PSA layer is preferably about 96% by weight or higher, more preferably about 97% by weight or higher, possibly about 98% by weight or higher, or even about 99% by weight or higher (e.g. about 99.5% by weight or higher, or even about 99.8% by weight or higher). To obtain aimed properties (e.g. anti-corrosive properties, tight adhesion, etc.), the maximum base polymer content of the PSA layer can also be 99.5% by weight or less, 98.5% by weight or less, or even 97.5% by weight or less.

As used herein, the base polymer of a PSA layer refers to the polymer constituting the layer formed of PSA and serving as an adhesive component (or an “adhesive polymer”). The weight of base polymer in the PSA layer includes that of any crosslinking agent bonded to the base polymer via crosslinking reaction. Similarly, with respect to a base polymer obtained by emulsion polymerization using a reactive surfactant, the weight of base polymer includes that of the reactive surfactant incorporated in the base polymer after the polymerization reaction.

The type of base polymer is not particularly limited. Accordingly, the type of PSA forming the PSA layer disclosed herein is not particularly limited. The PSA layer may be formed from a PSA composition comprising, as the base polymer, one, two or more species selected among various polymers (adhesive polymers), such as acrylic, polyester-based, urethane-based, polyether-based, rubber-based, silicone-based, polyamide-based, and fluorinated polymers. The art disclosed herein can be preferably made, for instance, as a PSA sheet comprising an acrylic PSA layer or a rubber-based PSA layer.

The “acrylic PSA layer” here refers to a PSA layer comprising an acrylic polymer as the base polymer. Similarly, the “rubber-based PSA layer” refers to a PSA layer comprising a rubber-based polymer as the base polymer. The “acrylic polymer” refers to a polymer whose primary monomer (the primary component among the monomers, i.e. a component that accounts for 50% by weight or more of the total amount of the monomers forming the acrylic polymer) is a monomer having at least one (meth)acryloyl group per molecule. Such a monomer may be referred to as an “acrylic monomer” hereinafter. As used herein, the “(meth)acryloyl group” comprehensively refers to acryloyl group and methacryloyl group. Similarly, the “(meth)acrylate” comprehensively refers to acrylate and methacrylate.

Acrylic Polymer

A preferable example of the acrylic polymer is a polymer of a starting monomer mixture that comprises an alkyl (meth)acrylate (or a monomer A hereinafter) and may further comprise another monomer (or a monomer B hereinafter) that is copolymerizable with the alkyl (meth)acrylate. The acrylic polymer typically has a monomer unit composition corresponding to the monomer composition of the starting monomer mixture.

A preferable monomer A is an alkyl (meth)acrylate represented by the next general formula (1):

CH₂=C(R¹)COOR²   (1)

Here, R¹ in the formula (1) is a hydrogen atom or a methyl group. R² is an alkyl group having 1 to 20 carbon atoms. Hereinafter, such a range of the number of carbon atoms may be indicated as “C₁₋₂₀.” From the standpoint of the polymerization reactivity, polymerization stability, etc., an alkyl (meth)acrylate wherein R² is a C₁₋₁₆ alkyl group is preferable, and an alkyl (meth)acrylate wherein R² is a C₁₋₁₂ (typically C₁₋₁₀, e.g. C₁₋₈) alkyl group is more preferable.

Examples of an alkyl (meth)acrylate with R² being a C₁₋₂₀ alkyl group include methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, s-butyl (meth)acrylate, t-butyl (meth)acrylate, pentyl (meth)acrylate, isopentyl (meth)acrylate, hexyl (meth)acrylate, heptyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, n-octyl (meth)acrylate, isooctyl (meth)acrylate, n-nonyl (meth)acrylate, isononyl (meth)acrylate, n-decyl (meth)acrylate, isodecyl (meth)acrylate, undecyl (meth)acrylate, lauryl (meth)acrylate, tridecyl (meth)acrylate, tetradecyl (meth)acrylate, pentadecyl (meth)acrylate, hexadecyl (meth)acrylate, heptadecyl (meth)acrylate, octadecyl (meth)acrylate, nonadecyl (meth)acrylate, eicosyl (meth)acrylate, etc. These alkyl (meth)acrylates can be used solely as one species or in a combination of two or more species.

The art disclosed herein can be preferably implemented in an embodiment where the alkyl acrylate as the monomer A accounts for about 50% by weight or more (more preferably about 75% by weight or more, yet more preferably about 90% by weight or more, e.g. about 95% by weight or more) of the total monomer content. As the alkyl acrylate, an alkyl acrylate with R² in the formula (1) being a C₁₋₂₀ alkyl group is preferable and an alkyl acrylate with R² being a C₁₋₁₂ (more preferably C₁₋₁₀, particularly preferably C₁₋₈) alkyl group is more preferable. The art disclosed herein can also be preferably implemented in an embodiment where the alkyl acrylate has a C₂₋₈ (typically C₄₋₈) alkyl group for R² in the formula (1). For the alkyl acrylate, solely one species or a combination of two or more species can be used. When two or more species of alkyl acrylate are used, to adjust the acrylic polymer's Tg to the most adequate range, etc., an alkyl acrylate A1 with R² being a C₄₋₂₀ (more preferably C₄₋₁₀, or yet more preferably C₄₋₈) alkyl group and an alkyl acrylate A2 with R² being a C₁₋₃ (more preferably C₁₋₂, e.g. C₂) alkyl group can be used together. In this embodiment, the alkyl acrylate A1 to alkyl acrylate A2 weight ratio (A1:A2) is not particularly limited. It is usually about 5:95 to 95:5, or suitably about 10:90 to 90:10, for instance, about 15:85 to 85:15.

In some preferable embodiments, the monomers include one, two or more species of alkyl methacrylate as the monomer A. With the use of the alkyl methacrylate, the base polymer can be preferably designed so as to exhibit desirable PSA properties. As the alkyl methacrylate, an alkyl methacrylate with R² in the formula (1) being a C₁₋₁₀ alkyl group is preferable and an alkyl methacrylate with R² being a C₁₋₄ (more preferably C₁ or C₂₋₄) alkyl group is more preferable. The alkyl methacrylate can be preferably used in combination with an alkyl acrylate. When an alkyl methacrylate and an alkyl acrylate are used together, with one, two or more species of alkyl methacrylate (e.g. C₂₋₄ alkyl methacrylate) having a weight C_(AM) and one, two or more species of alkyl acrylate having a weight C_(AA), their ratio (C_(AM):C_(AA)) is not particularly limited. In some embodiments, it is about 1:9 to 9:1, suitably about 2:8 to 8:2, preferably about 3:7 to 7:3, or more preferably about 4:6 to 6:4. In other embodiments, the ratio of the weight C_(AM) of the alkyl methacrylate (e.g. C₁ alkyl methacrylate, i.e. methyl methacrylate (MMA)) in the total amount (C_(AM)+C_(AA)) of the alkyl (meth)acrylate is about 30% by weight or lower, suitably about 20% by weight or lower, preferably about 10% by weight or lower, or more preferably about 7% by weight or lower. On the other hand, the lower limit is about 0.1% by weight or higher, suitably about 1% by weight or higher, or preferably about 2% by weight or higher (e.g. about 3% by weight or higher).

The art disclosed herein can be implemented in an embodiment where the monomers are essentially free of an alkyl methacrylate as the monomer A. In an embodiment using an alkyl methacrylate, it can be implemented, for instance, in an embodiment free of a C₁₋₃ alkyl methacrylate (typically MMA).

In other preferable embodiments, the acrylic polymer is a polymer of a starting monomer mixture that comprises an alkyl acrylate (or a monomer m_(A), hereinafter) having 4 to 9 alkyl carbon atoms and may further comprise other monomer(s) as necessary. The acrylic polymer can be synthesized by polymerizing a starting monomer mixture having the corresponding composition (monomer composition) by a known method.

Non-limiting examples of monomer m_(A) include n-butyl acrylate (BA), n-hexyl acrylate, 2-ethylhexyl acrylate (2EHA), n-octyl acrylate, n-nonyl acrylate and isononyl acrylate (iNA). A preferable alkyl acrylate includes a —(CH₂)₃— structure in the alkyl group and a more preferable alkyl acrylate includes a —(CH₂)₄— structure in the alkyl group. Favorable examples of monomer m_(A) include 2EHA, BA and iNA. For the monomer mA, solely one species or a combination of two or more species can be used.

The amount of monomer m_(A) used typically accounts for 40% by weight or more of the starting monomers used in the synthesis of acrylic polymer. From the standpoint of the flexibility and enhanced low-temperature properties of the PSA layer comprising the acrylic polymer, it is advantageous that the monomer composition includes at least 40% monomer m_(A) by weight. From such a standpoint, the ratio of monomer m_(A) is preferably 45% by weight or higher, or possibly 50% by weight or higher (e.g. above 50% by weight). When the monomer composition includes 99.9% or less monomer m_(A) by weight, the PSA layer comprising the acrylic polymer is likely to have suitable cohesion. From such a standpoint, the ratio of monomer m_(A) is preferably 95% by weight or lower, more preferably 90% by weight or lower, or possibly 85% by weight or lower. In some embodiments, the ratio of monomer m_(A) can be below 80% by weight or even below 70% by weight (e.g. 65% by weight or lower).

The acrylic polymer in the art disclosed herein may have a monomer composition comprising a carboxy group-containing monomer (or a monomer m_(B), hereinafter) as the monomer B. In other words, a monomer m_(B) may be copolymerized in the acrylic polymer. The monomer m_(B) may be useful in introducing crosslinking points into the acrylic polymer and increasing the cohesive strength of the PSA layer comprising the acrylic polymer. The monomer m_(B) may help enhance the anti-PSA-residue properties by increasing the tightness of adhesion (anchoring) between the acrylic polymer-containing PSA layer and the substrate layer. Examples of the carboxy group-containing monomer include ethylenically unsaturated monocarboxylic acids such as acrylic acid, methacrylic acid, crotonic acid, isocrotonic acid, carboxyethyl (meth)acrylate, carboxypentyl (meth)acrylate and 1-[2-(methacryloyloxy)ethyl] succinate; ethylenically unsaturated dicarboxylic acids such as maleic acid, fumaric acid, itaconic acid and citraconic acid, and anhydrides thereof (e.g. maleic anhydride, itaconic anhydride, etc.). These carboxy group-containing monomers can be used singly as one species or in a combination of two species.

When using a monomer m_(B), the ratio of monomer m_(B) in the monomer composition is not particularly limited. The ratio of monomer m_(B) can be, for instance, 0.05% by weight or higher, or typically suitably 0.1% by weight or higher. From the standpoint of the polymerization stability and dispersion stability, it is preferably 0.5% by weight or higher, or more preferably 1% by weight or higher. From the standpoint of inhibiting an excessive increase in peel strength (adhesive strength), the ratio of monomer m_(B) is suitably about 20% by weight or lower (preferably about 10% by weight or lower, typically about 7% by weight or lower), more preferably about 5% by weight or lower, or yet more preferably about 4% by weight or lower (e.g. 3% by weight or lower). When the adherend comprises a metal, the usage of carboxy group-containing monomer is preferably limited.

In the acrylic polymer in the art disclosed herein, for purposes such as adjusting the glass transition temperature (Tg) of the acrylic polymer, an alkyl (meth)acrylate (or a monomer mc, hereinafter) may be copolymerized as the monomer A, the alkyl (meth)acrylate having 4 to 20 alkyl carbon atoms and having a homopolymer Tg of −50° C. or higher. Here, a monomer belonging to the monomer m_(A) is not categorized as a monomer m_(C). For the monomer m_(C), solely one species or a combination of two or more species can be used. A preferable monomer mc has a homopolymer Tg in the range between −40° C. and +60° C. and a more preferable monomer mc in the range between −30° C. and +40° C. (e.g. between −20° C. and +30° C.). Non-limiting examples of a monomer mc that can be preferably used include n-butyl methacrylate (BMA), 2-ethylhexyl methacrylate and lauryl acrylate.

When using a monomer m_(C), the ratio of monomer m_(C) in the monomer composition is not particularly limited. The ratio of monomer m_(C) can be, for instance, 1% by weight or higher, 5% by weight or higher, 10% by weight or higher, or even 15% by weight or higher. In some embodiments, from the standpoint of obtaining greater effect of monomer m_(C), the ratio of monomer m_(C) in the monomer composition can also be 20% by weight or higher, 25% by weight or higher, 30% by weight or higher, or even 35% by weight or higher (e.g. 40% by weight or higher). The ratio of monomer m_(C) in the monomer composition can be below 60% by weight. From the standpoint of the flexibility and enhanced low-temperature properties, it is suitably about 55% by weight or lower (typically about 60% by weight or lower, e.g. about 46% by weight or lower). In some embodiments, the ratio of monomer mc can be below 30% by weight, below 20% by weight, or even below 10% by weight (e.g. below 5% by weight, or even below 1% by weight).

In the acrylic polymer in the art disclosed herein, as necessary, other monomer(s) (monomer m_(D)) can be copolymerized as the monomer A besides monomers m_(A), ma and m_(C). As the monomer m_(D), among various monomers copolymerizable with monomer m_(A), solely one species or a combination of two or more species can be used.

As the monomer m_(D), for instance, it is possible to use an alkyl (meth)acrylate (monomer m_(D1)) represented by the formula (1) shown earlier (while excluding any species considered a monomer m_(A) or monomer mc). Specific examples of the alkyl (meth)acrylate that can be used as monomer m_(D1) include methyl (meth)acrylate, ethyl (meth)acrylate and propyl (meth)acrylate. For the monomer m_(D1), solely one species or a combination of two or more species can be used.

Examples of compounds that can be used as the monomer m_(D) may include a functional group-containing monomer (monomer B) such as those described below. These functional group-containing monomers may be useful for introducing crosslinking points into the acrylic polymer or for increasing the cohesive strength of the acrylic polymer.

Hydroxy group-containing monomers: e.g. hydroxyalkyl (meth)acrylates such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 6-hydroxyhexyl (meth)acrylate, 8-hydroxyoctyl (meth)acrylate, 10-hydroxydecyl (meth)acrylate, 12-hydroxyauryl (meth)acrylate and (4-hydroxymethylcyclohexyl)methyl acrylate; unsaturated alcohols such as vinyl alcohol and allyl alcohol.

Amide group-containing monomers: e.g. (meth)acrylamide, N,N-dimethyl (meth)acrylamide, N,N-diethyl (meth)acrylamide, N,N-dimethylaminopropyl (meth)acrylamide, N-butyl (meth)acrylamide, N-methylol (meth)acrylamide, N-methylolpropane (meth)acrylamide, N-methoxymethyl (meth)acrylamide, N-butoxymethyl (meth)acrylamide and diacetone (meth)acrylamide;

Imide group-containing monomers: e.g. N-isopropylmaleimide, N-cyclohexylmaleimide, itaconimide;

Amino group-containing monomers: e.g. aminoethyl (meth)acrylate, N,N-dimethylaminoethyl (meth)acrylate, N,N-dimethylaminopropyl (meth)acrylate, t-butylaminoethyl (meth)acrylate;

Epoxy group-containing monomers: e.g. glycidyl (meth)acrylate, methylglycidyl (meth)acrylate, allyl glycidyl ether;

Cyano group-containing monomers: e.g. acrylonitrile, methacrylonitrile;

Keto group-containing monomers: e.g. diacetone (meth)acrylamide, diacetone (meth)acrylate, vinyl methyl ketone, vinyl ethyl ketone, allyl acetoacetate, vinyl acetoacetate;

Monomers having nitrogen atom-containing rings: e.g. N-vinyl-2-pyrrolidone, N-methylvinylpyrrolidone, N-vinylpyridine, N-vinylpiperidone, N-vinylpyrimidine, N-vinylpiperazine, N-vinylpyrazine, N-vinylpyrrole, N-vinylimidazole, N-vinyloxazole, N-vinylmorpholine, N-vinylcaprolactam, N-(meth)acryloyl morpholine, N-(meth)acryloylpyrrolidone;

Alkoxysilyl group-containing monomers: e.g. (3-(meth)acryloxypropyl)trimethoxysilane, (3-(meth)acryloxypropyl)triethoxysilane, (3-(meth)acryloxypropyl)methyldimethoxysilane, (3-(meth)acryloxypropyl)methyldiethoxysilane.

Other examples of the compound that can be used as the monomer m_(D) include vinyl ester-based monomers such as vinyl acetate and vinyl propionate; aromatic vinyl compounds such as styrene, substituted styrenes (α-methylstyrene, etc.) and vinyltoluene; non-aromatic ring-containing (meth)acrylates such as cyclohexyl (meth)acrylate, t-butylcyclohexyl (meth)acrylate, cyclopentyl di(meth)acrylate and isobornyl (meth)acrylate; aromatic ring-containing (meth)acrylates such as aryl (meth)acrylates (e.g. phenyl (meth)acrylate, benzyl (meth)acrylate), aryloxyalkyl (meth)acrylate (e.g. phenoxyethyl (meth)acrylate), arylalkyl (meth)acrylate (e.g. benzyl (meth)acrylate); olefinic monomers such as ethylene, propylene, isoprene, butadiene and isobutylene; chlorine-containing monomers such as vinyl chloride and vinylidene chloride; isocyanate group-containing monomers such as 2-(meth)acryloxyethylisocyanate; alkoxy group-containing monomers such as methoxymethyl (meth)acrylate and ethoxyethyl (meth)acrylate; vinyl etherbased monomers such as methyl vinyl ether, ethyl vinyl ether and isobutyl vinyl ether.

Yet other examples of the compound that can be used as the monomer m_(D) include polyfunctional monomers. Specific examples of polyfunctional monomers include compounds having two or more (meth)acryloyl groups per molecule such as 1,6-hexanediol di(meth)acrylate, ethylene glycol di(meth)acrylate, pentaerythritol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, dipentaerythritol hexa(meth)acrylate and methylene bisacrylamide. When using such a polyfunctional monomer, its amount used is not particularly limited. It is suitably 2% by weight or less (more preferably 1% by weight or less) of the total monomer content.

The monomer m_(D) is used in an amount of no more than 40% by weight of the monomer composition, preferably no more than 20% by weight, or more preferably no more than 10% by weight. The monomer mu may not be used. The art disclosed herein can be preferably practiced in an embodiment where the amount of monomer m_(D) used is 0% or more and below 5% by weight of the monomer composition. Here, that the amount of monomer mu used is 0% by weight means that no monomer m_(D) is used at least intentionally.

The ratio of monomer A (alkyl (meth)acrylate) in the total monomer content can be, but is not particularly limited to, for instance, about 50% by weight or greater; it is suitably about 60% by weight or greater, preferably about 70% by weight or greater, more preferably about 80% by weight or greater, or yet more preferably about 85% by weight or greater. With the inclusion of the monomer A in a prescribed amount, a PSA sheet having desirable adhesive properties can be favorably obtained. The art disclosed herein can be preferably implemented, for instance, in some embodiments where the monomer A content in the total monomer content is about 90% by weight or greater. In an embodiment, the monomer A content can be about 95% by weight or greater, or even about 97% by weight or greater. The PSA composition comprising such an acrylic polymer as the base polymer can be advantageous from the standpoint of the weatherability of the PSA layer formed therefrom (and also the PSA sheet having the PSA layer). In an embodiment using a monomer A and a monomer B together, from the standpoint of suitably obtaining the effects of the monomer B, the monomer A content in the total monomer content can be, for instance, 99.9% by weight or less; it is preferably 99.5% by weight or less, more preferably 99% by weight or less, or about 97% by weight or less (e.g. 95% by weight or less).

When an aforementioned functional group-containing monomer is copolymerized in the acrylic polymer, the ratio of the functional group-containing monomer to all the monomers forming the acrylic polymer is preferably about 0.1% by weight or higher (typically about 0.5% by weight or higher, e.g. about 1% by weight or higher), and preferably about 40% by weight or lower (typically about 30% by weight or lower, e.g. about 20% by weight or lower). When a hydroxy group-containing monomer is copolymerized with the acrylic monomer, from the standpoint of obtaining desirable cohesive strength, the ratio of hydroxy group-containing monomers to the total monomer content is suitably about 0.001% by weight or higher (typically about 0.01% by weight or higher, e.g. about 0.1% by weight or higher), preferably about 1% by weight or higher, or more preferably about 3% by weight or higher; it is preferably about 10% by weight or lower (typically about 7% by weight or lower, e.g. about 5% by weight or lower).

Rubber-Based Polymer

In other preferable embodiments, the PSA layer can be a rubber-based PSA layer. Examples of the base polymer include natural rubber; styrene-butadiene rubber (SBR); polyisoprene; butene-based polymer comprising a butene (1-butene or cis- or trans-2-butene) and/or 2-methylpropene (isobutylene) as the primary monomer(s); A-B-A block copolymer rubber and a hydrogenation product thereof, e.g. styrene-butadiene-styrene block copolymer rubber (SBS), styrene-isoprene-styrene block copolymer (SIS), styrene-isobutylene-styrene block copolymer rubber (SIBS), styrene-vinyl-isoprene-styrene block copolymers (SVIS), hydrogenated SBS (styrene-ethylene/butylene-styrene block copolymer (SEGS)), and hydrogenated SIS (styrene-ethylene-propylene-styrene block copolymers (SEPS)). These rubber-based polymers can be used singly as one species or in a combination of two or more species.

Tg of Base Polymer

The Tg value of the PSA layer's base polymer (an acrylic polymer in case of an acrylic PSA layer being used) is not particularly limited in Tg. The Tg of the base polymer can be, for instance, about 0° C. or lower. In some preferable embodiments, the base polymer of the PSA layer has a Tg of about −5° C. or lower. According to a base polymer having such a Tg, a PSA layer that tightly adheres to adherend can be favorably formed. In an embodiment where the base polymer has a Tg of about −15° C. or lower (more preferably about −20° C. or lower, e.g. about −25° C. or lower), greater effects can be obtained. In another preferable embodiment, from the standpoint of the adhesion to adherends, the base polymer of the PSA layer has a Tg of about −35° C. or lower, more preferably about −40° C. or lower, or yet more preferably about −45° C. or lower (e.g. below −50° C., even about −52° C. or lower, or about −55° C. or lower). The Tg of the base polymer is suitably −70° C. or higher. From the standpoint of the cohesion of the PSA, etc., it is preferably about −65° C. or higher, more preferably −60° C. or higher, yet more preferably about −55° C. or higher (e.g. about −50° C. or higher), or possibly even about −35° C. or higher. The base polymer's Tg can be adjusted by suitably changing the monomer composition (i.e. the monomer species used in the synthesis of the polymer and their ratio).

In the present description, the Tg of a polymer refers to the value determined by the Fox equation based on the Tg values of homopolymers of the respective monomers forming the polymer and the weight fractions (copolymerization ratio by weight) of the monomers. As shown below, the Fox equation is a relational expression between the Tg of a copolymer and glass transition temperatures Tgi of homopolymers of the respective monomers constituting the copolymer.

1/Tg=Σ(Wi/Tgi)

In the Fox equation, Tg represents the glass transition temperature (unit: K) of the copolymer, Wi the weight fraction (copolymerization ratio by weight) of a monomer i in the copolymer, and Tgi the glass transition temperature (unit: K) of homopolyrner of the monomer i.

For the glass transition temperatures of homopolymers used for determining the Tg, the values found in known documents are used. For instance, with respect to the monomers listed below, for the glass transition temperatures of their homopolymers, the following values are used:

2-ethylhexyl acrylate −70° C. n-hexyl acrylate −65° C. n-octylacrylale −65° C. isononyl acrylate −60° C. n-nonyl acrylate −58° C. n-butyl acrylate −55° C. ethyl acrylate −20° C. lauryl acrylate 0° C. 2-ethylhexyl methacrylate −10° C. methyl acrylate 8° C. n-butyl methacrylate 20° C. methyl methacrylate 105° C. acrylic acid 106° C. methacrylic acid 228° C. vinyl acetate 32° C. styrene 100° C.

With respect to the Tg values of homopolymers other than the examples listed above, the values given in Polymer Handbook (3rd edition, John Wiley & Sons, Inc., Year 1989) are used. With respect to a monomer for which two or more values are listed in the Polymer Handbook, the highest value is used. When no values are given in the Polymer Handbook, values obtained by the measurement method described in Japanese Patent Application Publication No. 2007-51271 are used.

Synthesis of Base Polymer

The method for obtaining the base polymer (e.g. an acrylic polymer) is not particularly limited. Known polymerization methods can be suitably employed, such as solution polymerization, emulsion polymerization, bulk polymerization, and suspension polymerization. Alternatively, it is also possible to employ photopolymerization involving irradiation of light such as UV (typically carried out in the presence of a photopolymerization initiator) and active energy ray irradiation polymerization such as radiation polymerization involving irradiation of radioactive rays such as B rays and y rays. In some preferable embodiments, the base polymer (e.g. acrylic polymer) is obtained by emulsion polymerization of a starting monomer mixture having an aforementioned composition. As the monomer supply method in, for instance, emulsion polymerization, a suitable method can be employed among the all-at-once method where all the starting monomer mixture is supplied in one portion, gradual supply method, portion-wise supply method, etc. An emulsion of some or all of the monomers pre-mixed with water and a surfactant can be supplied to the polymerization vessel.

The polymerization temperature can be suitably selected in accordance with the monomer species, the solvent species, and the polymerization initiator species used, etc. The polymerization temperature is suitably about 20° C. or higher, preferably about 40° C. or higher, more preferably about 50° C. or higher; it can also be about 60° C. or higher, about 65° C. or higher, or even about 70° C. or higher. The polymerization temperature is suitably about 170° C. or lower (typically about 140° C. or lower), or preferably about 95° C. or lower (e.g. about 85° C. or lower). In emulsion polymerization, the polymerization temperature is preferably about 95° C. or lower (e.g. about 85° C. or lower).

The solvent (polymerization solvent) used for solution polymerization can be suitably selected among heretofore known organic solvents. For instance, aromatic compounds (typically aromatic hydrocarbons) such as toluene, acetic acid esters such as ethyl acetates, aliphatic and alicyclic hydrocarbons such as hexane and cyclohexane are preferably used.

Examples of the polymerization initiator include, but not limited to, azo-based initiators, peroxide-based initiators and redox-based initiators by the combination of a peroxide and a reducing agent. For the polymerization initiator, solely one species or a combination of two or more species can be used.

Examples of azo-based initiators include 2,2′-azobisisobutyronitrile, 2,2′-azobis[N-(2-carboxyethyl)-2-methylpropionamidine]hydrate, 2,2′-azobis(2-methylpropionamidine) disulfate salt, 2,2′-azobis(2-methylpropionamidine) dihydrochloride, 2,2′-azobis[2-(5-methyl-2-imidazolin-2-yl)propane] dihydrochloride and 2,2′-azobis(N,N-dimethylene isobutylamidine) dihydrochloride.

Examples of the peroxide-based initiator include persulfates such as potassium persulfate and ammonium persulfate; benzoyl peroxide, t-butyl hydroperoxide, di-t-butyl peroxide, dicumyl peroxide, dilauroyl peroxide, di-n-octanoyl peroxide, di(4-methylbenzoyl) peroxide, t-butyl peroxybenzoate, t-butyl peroxyisobutyrate, t-hexyl peroxypivalate, t-butyl peroxypivalate, di(2-ethylhexyl) peroxydicarbonate, di(4-t-butylcyclohexyl) peroxydicarbonate, di-sec-butyl peroxydicarbonate, t-butyl peroxyneodecanoate, 1,1,3,3-tetramethyl butylperoxy-2-ethylhexanoate, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, 1,1-bis(t-butylpemxykyclododecane, 1,1-bis(t-hexylperoxy)cyclohexane and hydrogen peroxide.

Examples of the redox-based initiator include a combination of a peroxide and ascorbic acid (combination of hydrogen peroxide and ascorbic acid, etc.), a combination of a peroxide and an iron(II) salt (combination of hydrogen peroxide and an iron(II) salt, etc.), and a combination of a persulfate salt and sodium hydrogen sulfite.

The polymerization initiator can be used in an amount suitably selected in accordance with the type of initiator, monomer species (monomer composition), polymerization conditions, etc. The amount of polymerization initiator used is, for instance, about 0.0005 part to 1 part by weight to 100 parts by weight of the starting monomers; and it is suitably in the range between about 0.001 part and 0.5 part by weight; preferably in the range between 0.002 part and 0.3 part by weight; and more preferably in the range between 0.005 part and 0.1 part by weight.

In the emulsion polymerization, as necessary, various heretofore known chain transfer agents (which can be considered also as a molecular weight-adjusting agent or polymerization degree-adjusting agent) can be used. For the chain transfer agent, solely one species or a combination of two or more species can be used. As the chain transfer agent, mercaptans can be preferably used, such as n-dodecyl mercaptan, t-dodecyl mercaptan, glycidyl mercaptan, 2-merraptoethanol, mercaptoacetic acid, thioglycolic acid, 2-ethylhexyl thioglycolate and 2,3-dimercapto-1-propanol. When using a chain transfer agent, its amount used to 100 parts by weight of the total monomer content is, for instance, possibly about 0.01 to 1 part by weight, preferably 0.02 to 0.1 part by weight, or more preferably 0.03 to 0.07 part by weight. The art disclosed herein can be preferably practiced in an embodiment without use of a chain transfer agent.

The surfactant (emulsifier) used in emulsion polymerization is not particularly limited. Commonly-known anionic surfactants, nonionic surfactants and the like can be used. A surfactant having a radically polymerizable functional group can also be used. Hereinafter, the surfactant having a radically polymerizable functional group is referred to as a reactive (polymerizing) surfactant. In contrast to this, a general surfactant free of a radically polymerizable functional group may be referred to as a non-reactive (non-polymerizing) surfactant. For the surfactant, solely one species or a combination of two or more species can be used.

Examples of the non-reactive surfactant include anionic emulsifiers such as sodium lauryl sulfate, ammonium lauryl sulfate, sodium dodecyl benzene sulfonate, sodium polyoxyethylene lauryl sulfate, sodium polyoxyethylene alkyl ether sulfates, ammonium polyoxyethylene alkyl phenyl ether sulfates, sodium polyoxyethylene alkyl phenyl ether sulfates, and sodium polyoxyethylene alkyl sulfosuccinates; and non-ionic emulsifiers such as polyoxyethylene alkyl ethers, polyoxyethylene alkyl phenyl ethers, polyoxyethylene aliphatic acid esters, and polyoxyethylene-polyoxypropylene block polymers.

The reactive surfactant is not particularly limited as far as it has a radically polymerizable functional group. For instance, the reactive surfactant may have a structure such that a radically polymerizable functional group is incorporated in an aforementioned anionic surfactant or nonionic surfactant. Examples of the radically polymerizable functional group include vinyl group, propenyl group, isopropenyl group, vinyl ether group (vinyloxy group), and allyl ether group (allyloxy group). The concept of propenyl group referred to herein encompasses 1-propenyl group (CH₃—CH═CH—) and 2-propenyl group (CH₂═CH—CH₂— which may be called allyl group).

Examples of an anionic reactive surfactant include polyoxyethylene (allyloxymethyl) alkyl ether sulfates (e.g. ammonium salts), polyoxyethylene nonyl propenyl phenyl ether sulfates (e.g. ammonium salts), alkyl allyl sulfosuccinates (e.g. sodium salts), methacryloxy polyoxypropylene sulfuric acid ester salts (e.g. sodium salts), and polyoxyalkylene alkenyl ether sulfates (e.g. an ammonium salt having an isopropenyl group as the terminal alkenyl group). When the anionic reactive surfactant is forming a salt, the salt can be, for instance, a metal salt such as sodium salt or a non-metal salt such as ammonium salt and amine salt.

Examples of a nonionic reactive surfactant include polyoxyethylene nonyl propenyl phenyl ether.

Although no particular limitations are imposed, in some embodiments, a reactive surfactant having an oxyethylene chain can be preferably used. The oxyethylene chain refers to a structure of repeating oxyethylene units, that is, a structural moiety represented by —(C₂H₄O)_(n)—, with n indicating the number of repeats of the oxyethylene unit. For instance, in a preferable reactive surfactant, the number of repeats, n, is about 5 to 30 (e.g. 8 to 25).

From the standpoint of the polymerization stability during the emulsion polymerization, in some embodiments, it is preferable to use a reactive surfactant having a propenyl group. A preferable reactive surfactant has a propenyl group and also an oxyethylene chain.

From the standpoint of the emulsifying ability, etc., in some embodiments, an anionic reactive surfactant (e.g. an anionic reactive surfactant having an oxyethylene chain) can be preferably used. When the anionic reactive surfactant is in a salt form, as the salt, a non-metal salt is preferable. In particular, an ammonium salt is preferable.

When using a nonionic reactive surfactant, more favorable results can be obtained by the combined use with other surfactant(s), such as an anionic reactive surfactant, anionic non-reactive surfactant and nonionic non-reactive surfactant.

By carrying out emulsion polymerization of the starting monomer mixture in the presence of a reactive surfactant having a radically polymerizable functional group, the reactive surfactant may undergo a reaction to be incorporated into the base polymer (e.g. acrylic polymer). The reactive surfactant incorporated in the base polymer is less likely to bleed out to the PSA layer surface because its move within the PSA layer is limited. Accordingly, the use of the reactive surfactant can reduce bleed-out of a low molecular weight compound to the PSA layer surface. This is preferable from the standpoint of the low-contaminating properties. From the standpoint of obtaining greater low-contaminating properties, it is preferable to apply an embodiment using solely a reactive surfactant as the surfactant for emulsion polymerization.

The amount of surfactant used is preferably about 0.1 part by weight or greater (e.g. about 0.5 part by weight or greater) to 100 parts by weight of the total monomer content; and it is suitably about 10 parts by weight or less (e.g. about 5 parts by weight or less) or possibly about 4 parts by weight or less to 100 parts by weight of the total monomer content. From the standpoint of the adhesive properties, lower possibilities of adherend surface contamination, etc., the amount of non-reactive surfactant used to 100 parts by weight of the total monomer content is about 5 parts by weight or less, suitably about 4 parts by weight or less, or preferably about 3 parts by weight or less (e.g. about 2 parts by weight or less).

The PSA layer of the PSA sheet disclosed herein can be formed from various forms of PSA compositions. Examples of forms of the PSA composition include a solvent-based PSA composition containing the PSA (adhesive component(s)) in an organic solvent, a water-dispersed PSA composition containing at least part of the PSA dispersed in an aqueous solvent, an active energy ray-curable PSA composition formulated so as to cure with active energy rays such as UV rays and radioactive rays to form PSA, and a hot-melt PSA composition which is applied in the molten state by heating and forms PSA when it cools to near room temperature.

From the standpoint of reducing environmental stress, a water-dispersed PSA composition can be preferably selected. A favorable example of the water-dispersed PSA composition is a water-dispersed PSA composition (water-dispersed acrylic PSA composition, typically an emulsion-based acrylic PSA composition) comprising an acrylic polymer as the base polymer. From the standpoint of the adhesive properties, a solvent-based PSA composition is preferable. A hot-melt PSA composition which does not require the use of solvent is advantageous for having excellent handling properties in manufacturing processes.

Crosslinking Agent

In the art disclosed herein, the PSA composition used to form the PSA layer preferably comprises a crosslinking agent. The PSA layer formed from a PSA composition comprising a base polymer and a crosslinking agent typically comprises a polymer (base polymer) crosslinked with the crosslinking agent. According to such a PSA layer, the surface hardness of the PSA layer can be suitably adjusted and good aged removability is readily obtained. The type of crosslinking agent used is not particularly limited and can be suitably selected from heretofore known crosslinking agents.

Specific examples of the crosslinking agent include oxazoline-based crosslinking agents, aziridine-based crosslinking agents, isocyanate-based crosslinking agents, epoxy-based crosslinking agents, melamine-based crosslinking agents, peroxide-based crosslinking agents, urea-based crosslinking agents, metal alkoxide-based crosslinking agents, metal chelate-based crosslinking agents, metal salt-based crosslinking agents, carbodiimide-based crosslinking agents, hydrazine-based crosslinking agents, amine-based crosslinking agents, and silane coupling agents. These can be used solely as one species or in a combination of two or more species. For instance, it is preferable to use one, two or more species selected from a group consisting of oxazoline-based crosslinking agents, aziridine-based crosslinking agents, isocyanate-based crosslinking agents and epoxy-based crosslinking agents. In particular, oxazoline-based crosslinking agent, isocyanate-based crosslinking agent and epoxy-based crosslinking agent are more preferable. In an embodiment using a water-dispersed PSA composition, a preferable crosslinking agent is soluble or dispersible in water and a particularly preferable crosslinking agent is soluble in water (i.e. water-soluble).

As the oxazoline-based crosslinking agent, a species having one or more oxazoline groups per molecule can be used without particular limitations. In the water-dispersed PSA composition, it is preferable to use an oxazoline-based crosslinking agent that is soluble or dispersible in water (an aqueously soluble or aqueously dispersible oxazoline-based crosslinking agent).

The oxazoline group can be either 2-oxazoling group, 3-oxazoline group or 4-oxazoline group. A 2-oxazoline group-containing oxazoline-based crosslinking agent can be preferably used. As the oxazoline-based crosslinking agent, a water-soluble copolymer or a water-dispersed copolymer can be used, which is obtained by copolymerizing an addition-polymerizable oxazoline such as 2-vinyl-2-oxazoline, 2-vinyl-4-methyl-2-oxazoline, 2-vinyl-5-methyl-2-oxazoline, 2-isopropenyl-2-oxazoline, 2-isopropenyl-4-methyl-2-oxazoline and 2-isopropenyl-5-ethyl-2-oxazoline with other monomer(s).

Examples of a commercial oxazoline-based crosslinking agent include products of Nippon Shokubai Co., Ltd., under trade names EPOCROS WS-500, EPOCROS WS-700, EPOCROS K-2010E, EPOCROS K-2020E and EPOCROS K-2030E.

Examples of the aziridine-based crosslinking agent include trimethylolpropane tris[3-(1-aziridinyl)propionate] and trimethylolpropane tris[3-(1-(2-methyl)aziridinylpropionate)]. Examples of commercial aziridine-based crosslinking agent include product names CHEMITITE PZ-33 and CHEMITITE DZ-22E available from Nippon Shokubai Co., Ltd.

As an example of the isocyanate-based crosslinking agent, a bifunctional or higher polyfunctional isocyanate compound can be used. Examples include aromatic isocyanates such as tolylene diisocyanates, xylylene diisocyanate, polymethylene polyphenyl diisocyanate, tris(pisocyanatophenylithiophosphate, and diphenylmethane diisocyanate; alicyclic isocyanates such as isophorone diisocyanate; and aliphatic isocyanates such as hexamethylene diisocyanate. Commercial products include isocyanate adducts such as trimethylolpropane/tolylene diisocyanate trimer adduct (trade name CORONATE L available from Tosoh Corporation), trimethylolpropane/hexamethylene diisocyanate trimer adduct (trade name CORONATE HL available from Tosoh Corporation) and hexamethylene diisocyanate isocyanurate (trade name CORONATE HX available from Tosoh Corporation). In the water-dispersed PSA composition, it is preferable to use a water-soluble or water-dispersible isocyanate-based crosslinking agent. For instance, a watersoluble, water-dispersible or self-emulsifying isocyanate-based crosslinking agent can be preferably used. A so-called blocked isocyanate-based crosslinking agent can be preferably used as an isocyanate-based crosslinking agent.

As the epoxy-based crosslinking agent, a species having two or more epoxy groups per molecule can be used without particular limitations. An epoxy-based crosslinking agent having 3 to 5 epoxy groups per molecule is preferable. In the water dispersed PSA composition, it is preferable to use a water-soluble or water-dispersible epoxy-based crosslinking agent.

Specific examples of the epoxy-based crosslinking agent include N,N,N′,N′-tetraglycidyl-m-xylenediamine, 1,3-bis(N,N-diglycidylaminomethyl)cyclohexane, 1,6-hexanediol diglycidyl ether, polyethylene glycol diglycidyl ether, and polyglycerol polyglycidyl ether.

Commercial epoxy-based crosslinking agents include products of Mitsubishi Gas Chemical Co., Inc., under trade names TETRAD-X and TETRAD-C, a product of DIC Corporation under trade name EPICLON CR-5L, a product of Nagase ChemteX Corporation under trade name DENACOL EX-512, and a product of Nissan Chemical Industries, Ltd., under trade name TEPIC-G.

As the carbodiimide-based crosslinking agent, a low or high molecular weight compound having two or more carbodiimide groups can be used. In a water-dispersed PSA composition, it is preferable to use a water-soluble or water-dispersible carbodiimide-based crosslinking agent. Examples of commercial carbodiimide-based crosslinking agents include the CARBODILITE series such as the CARBODILITE V series (aqueous solutions) including CARBODILITE V-02, CARBODILITE V-02-L2, and CARBODILITE V-04; and the CARBODILITE E series (aqueous dispersions) including CARBODILITE E-01, CARBODILITE E-02, and CARBODILITE E-04 available from Nisshinbo Holdings, Inc.

The crosslinking agent content (the total amount of crosslinking agent) in the PSA composition disclosed herein is not particularly limited and can be suitably selected in view of the composition and the molecular weight of the base polymer so as to obtain favorable properties after crosslinked. While no particular limitations are imposed, the amount of the crosslinking agent used to 100 parts by weight of the base polymer (typically an acrylic polymer) is about 0.01 part by weight or greater, suitably about 0.1 part by weight or greater, or preferably about 1 part by weight or greater (e.g. about 2 parts by weight or greater). From the standpoint of the adhesion, etc., the amount of the crosslinking agent is suitably about 15 parts by weight or less (preferably about 10 parts by weight or less, e.g. about 5 parts by weight or less) to 100 parts by weight of the base polymer. From the standpoint of increasing the tightness of adhesion to adherends, it is preferably about 4 parts by weight or less, more preferably less than 3.5 parts by weight, or yet more preferably less than 3 parts by weight.

Phosphate

The PSA composition (and also the PSA layer) disclosed herein may include a phosphate. Examples of the phosphate include alkyl phosphates such as lauryl phosphate and a lauryl phosphate salt; and a phosphate having an oxyethylene chain and a salt thereof. The salt can be, for instance, sodium salts, potassium salts, barium salts and triethanolamine salts of these phosphates. In the following, unless otherwise noted, the “phosphate” includes a salt. In particular, it is more preferable to use a phosphate having an oxyethylene chain. The oxyethylene chain-containing phosphate can suppress the increase in adhesive strength with aging (i.e. increase the stability of the adhesive strength) while preventing penetration of corrosion-induced components such as water, an acid and a base at the interface between the PSA layer and the adherend surface. Here, the oxyethylene chain refers to a structural moiety in chain form that includes at least one ethylene oxide (EO) unit and may further include other oxyalkylene unit(s) (e.g. oxyalkylene unit(s) with about 3 to 6 carbon atoms). One favorable example of the oxyethylene chain-containing phosphate is a phosphate having an oxyethylene chain formed with an EO unit or repeats of this. For instance, a phosphate represented by the next general formula (a) or a salt thereof can be preferably used.

In the general formula (a), R¹ is —OH or —(OCH₂CH₂)_(n)OR³; R² represents —(OCH₂CH₂)_(m)OR⁴; n and m indicate the number of moles of EO added. The number of moles of EO added, n, is an integer between 1 and 30; it can be typically an integer between about 1 and 20, preferably an integer between about 1 and 10, for instance, an integer between about 1 and 8. The number of moles of EO added, n, is preferably an integer between about 1 and 6, or yet more preferably an integer between 1 and 4 (e.g. 2 and 4). In the general formula (a), the number of moles of EO added, m, can be typically about the same as the number of moles of EO added for n; n and m may be identical or different. R³ and R⁴ are mono-valent organic groups (typically hydrocarbon groups); for instance, each can be individually a group selected among an alkyl group, cycloalkyl group, aryl group, alkylaryl group, and arylalkyl group. R³ and R⁴ are preferably linear or branched alkyl groups, aryl groups or alkylaryl groups. R³ and R⁴ are individually an organic group with 1 to 30 carbon atoms, or possibly an organic group with 6 or more (preferably 8 or more, e.g., 11 or more) carbon atoms. In some preferable embodiments, R³ and R⁴ can be organic groups with 20 or fewer or preferably 18 or fewer, for instance, 15 or fewer carbon atoms. The phosphate salts represented by the general formula (a) can be, for instance, sodium salts, potassium salts, barium salts and triethanolamine salts of these phosphates. For the phosphate, solely one species or a combination of two or more species can be used.

Examples of the phosphates include polyoxyethylene alkylphosphoric acid esters such as polyoxyethylene tridecyl ether phosphate, polyoxyethylene lauryl ether phosphate, and polyoxyethylene octadecyl ether phosphate; and polyoxyethylene alkyl aryl phosphoric acid esters such as polyoxyethylene nonyl phenyl ether phosphate, polyoxyethylene octyl phenyl ether phosphate, polyoxyethylene dinonyl phenyl ether phosphate, and polyoxyethylene dioctyl phenyl ether phosphate. In some embodiments, a phosphoric acid ester having a molecular weight of 150 to 5000 can be preferably used.

The amount of phosphate used can be, for instance, about 0.05 part by weight or greater to 100 parts by weight of the base polymer (e.g. an acrylic polymer); it can be about 0.1 part by weight or greater, or also about 0.3 part by weight or greater (e.g. about 0.5 part by weight or greater). From the standpoint of leaving a lower degree of contamination on the adherend surface, the amount of phosphate used to 100 parts by weight of the base polymer (e.g. an acrylic polymer) is suitably about 5 parts by weight or less (e.g. 3 parts by weight or less), preferably about 2 parts by weight or less, or more preferably about 1 part by weight or less (e.g. 0.1 part by weight or less). The art disclosed herein can be preferably implemented in an embodiment where the PSA layer is essentially free of phosphates. According to the PSA layer free of phosphates or limited in phosphate content, adherend surface contamination is highly inhibited. For instance, when the surface of adherend (typically a metal such as an aluminum plate) is subjected to a process such as coating after removal of the PSA sheet adhered for a certain time period, defects (coating defects, etc.) can be avoided in the process.

Rust Inhibitor

The PSA layer disclosed herein may or may not comprise a rust inhibitor. The PSA layer according to some preferable embodiments may include a rust inhibitor. The rust inhibitor is not particularly limited and a suitable species is selected for its anti-rust effect relative to the adherend. Examples of the rust inhibitor include amine compounds (e.g. amine salts of organic and inorganic acids, triethanolamine, aliphatic acid-based amine compounds, alicyclic amines (in particular, polyoxyalkylene-containing alicyclic amines)), azole-based compounds (triazole-based compounds, etc.), nitrites, ammonium benzoate, ammonium phthalate, ammonium stearate, ammonium palmitate, ammonium oleate, ammonium carbonate, dicyclohexylaminebenzoic acid salts, urea, urotropin, thiourea, phenyl carbamate, and cyclohexylammonium-N-cyclohexyl carbamate (CHC). These rust inhibitors can be used singly as one species or in a combination of two or more species. Among them, amine-based compounds and azole-based compounds are preferable. The species of rust inhibitor is desirably selected in view of influences on the adhesive properties (e.g. possibilities to impair crosslinking reactions, etc.).

The amount of rust inhibitor (e.g. an amine-based compound (amine-based rust inhibitor) or an azole-based compound (azole-based rust inhibitor) is not particularly limited. It can be, for instance, 0.01% by weight or greater (typically 0.05% by weight or greater) of the PSA layer. From the standpoint of obtaining greater anti-corrosion effects, the amount may be 0.1% by weight or greater, 0.3% by weight or greater, even 0.5% by weight or greater, 1% by weight or greater, 1.5% by weight or greater, or even 2% by weight or greater (e.g. 2.5% by weight or greater). On the other hand, the rust inhibitor content is suitably below 5% by weight of the PSA layer. In some preferable embodiments, the rust inhibitor content of the PSA layer is 3% by weight or less. By limiting the usage of rust inhibitor to or below the prescribed values, good removability can be preferably maintained while inhibiting the increase in aged adhesive strength. The rust inhibitor content of the PSA layer is more preferably about 2% by weight or less, or possibly about 1.5% by weight or less (e.g. about 1.0% by weight or less). The art disclosed herein can be preferably implemented in an embodiment using essentially no rust inhibitor. According to the PSA layer free of rust inhibitor or limited in rust inhibitor content, adherend surface contamination is highly inhibited. For instance, when the surface of adherend (typically a metal such as an aluminum plate) is subjected to a process such as coating after removal of the PSA sheet adhered for a certain time period, defects (coating defects, etc.) can be avoided in the process. In an embodiment using a base polymer crosslinked with a crosslinking agent (e.g. an oxazoline-based crosslinking agent), from the standpoint of aged removability, it is particularly significant to limit the amount of rust inhibitor (e.g. an amine-based rust inhibitor) which may impair the crosslinking reaction of the crosslinking agent.

Other Components, etc.

The PSA layer can comprise, as necessary, a known tackifier such as a rosin-based tackifier, terpene-based tackifier and hydrocarbon-based tackifier. From the standpoint of avoiding an excessive increase in peel strength, the amount of tackifier is preferably about 5 parts by weight or less to 100 parts by weight of the base polymer, or more preferably about 1 part by weight or less. For the PSA sheet disclosed herein, the adhesive strength can be effectively controlled by the base polymer's composition and Tg as well as PSAs gel fraction, etc.; and therefore, the surface PSA sheet can be preferably made in an embodiment using no tackifier as well.

The PSA layer may comprise, as necessary, various optional additives generally known in the field of PSA compositions, such as viscosity-adjusting agent (viscosifier, etc.), crosslinking accelerator, plasticizer, softener, filler, anti-static agent, anti-aging agent, UV-absorber, antioxidant, photo-stabilizing agent and defoamer. In the PSA composition according to some embodiments, the amount of plasticizer such as aliphatic acid ester can be limited to, for instance, below 5 parts by weight (e.g. below 1 part by weight) to 100 parts by weight of base polymer. In typical, the PSA composition may be essentially free of a plasticizer. The art disclosed herein can be preferably implemented in an embodiment essentially free of an anti-static agent (possibly a conducting agent) such as an ionic compound (e.g. ionic liquid, alkaline metal salt). With respect to other matters about these various optional additives, various heretofore known additives and methods can be selected based on technical common knowledge. Because these additives do not characterize the present invention in particular, details are omitted.

In the art disclosed herein, in an embodiment using a water-dispersed PSA composition for PSA layer formation, the PSA composition can be prepared, for instance, by mixing an acrylic polymer dispersion in water obtained by emulsion polymerization using a surfactant with other components (e.g. crosslinking agent, etc) used as necessary. The water dispersion may comprise the aqueous liquid described later. For instance, it is possible to use a polymerization reaction mixture obtained by emulsion polymerization or the polymerization reaction mixture subjected as necessary to a treatment such as pH adjustment (e.g. neutralization), non-volatile content adjustment and viscosity adjustment. In typical, the emulsion's dispersion stability can be increased by pH adjustment in a suitable range (e.g. in the range between about pH 6 and pH 9) with addition of a neutralizer such as ammonia water.

While no particular limitations are imposed, from the standpoint of efficient drying, etc., the non-volatile content (NV) of the PSA composition can be about 20% by weight or higher (typically about 30% by weight or higher, preferably about 40% by weight or higher). From the standpoint of the ease of application, etc., it can be about 75% by weight or lower (typically about 70% by weight or lower, preferably about 60% by weight or lower).

Chloride Ion Amount

In the PSA layer disclosed herein, the chloride ion amount determined by hot water extraction is not limited to a specific range as long as the chloride ion amount per gram of PSA sheet is below the prescribed value. In some embodiments, the chloride ion amount per gram of PSA (layer) can be below 300 μg. According to the PSA layer having such a limited chloride ion amount, even in an embodiment prolongedly adhered to a metal adherend, metal corrosion tends to be prevented without adding an anti-corrosive such as rust inhibitor or with a limited usage thereof. The chloride ion amount per gram of PSA (layer) is suitably about 250 μg or less, or possibly about 200 μg or less (e.g. about 150 μg or less). By limiting the chloride ion amount of the PSA touching the adherend, even in an embodiment where the PSA sheet is applied to metal, metal corrosion of the adherend is likely to be prevented for a long period in the resulting configuration. In some embodiments, the chloride ion amount per gram of PSA (layer) can also be about 100 μg or less, about 95 μg or less, about 90 μg or less, about 85 μg or less, or even about 75 μg or less (about 70 μg or less). The minimum chloride ion amount per gram of PSA (layer) is ideally 0 μg; however, from the standpoint of the productivity, allowable level for practical use, etc., it can be about 1 μg or greater, about 10 μg or greater, or even about 50 μg or greater. In other embodiments, the chloride ion amount per gram of PSA (layer) can also be about 70 μg or greater, or even about 100 μg or greater (e.g. about 120 μg or greater). According to the art disclosed herein, by adding a limited amount of rust inhibitor to the PSA layer having such a chloride ion amount, good anti-metal-corrosive properties can be obtained without impairing the aged removability.

The chloride ion amount per gram of PSA (layer) is determined by the method described later in Examples. Alternatively, it can also be determined as follows: Chloride ions in the PSA sheet and the substrate layer constituting the PSA sheet are quantified by hot water extraction; from the difference in chloride ion amount between the two, the chloride ion amount from the PSA layer is determined; and the resulting value is converted to a per unit PSA (layer) value.

Gel Fraction

The PSA layer disclosed herein is not particularly limited in weight fraction (gel fraction) of ethyl acetate-insoluble portion; and it can be about 40% or higher. In some embodiments, the gel fraction of the PSA layer is suitably about 50% or higher, preferably about 60% or higher, or more preferably about 70% or higher (e.g. about 80% or higher). The gel fraction of the PSA layer can be, for instance, about 90% or higher (e.g. about 95% or higher). With increasing gel fraction, the PSA's cohesion tends to increase while the aged adhesive strength tends to be reduced. The maximum gel fraction is theoretically 1.00%. In some embodiments, the gel fraction can be, for instance, about 98% or lower, or even about 92% or lower (e.g. about 85% or lower). The gel fraction can be adjusted through, for instance, the base polymer composition and its content (percentage), the base polymer's polymerization method and conditions, the base polymer's molecular weight, the use of crosslinking agent as well as its selected type and amount used if any, species of additives, etc. The gel fraction is determined by the method described below. The same method is used for the working examples described later.

Degree of Swelling

The degree of swelling of the PSA layer disclosed herein is not particularly limited. It can be about 30-fold or less. From the standpoint of the aged removability, etc., the degree of swelling is suitably about 25-fold or less, preferably about 22-fold or less, or more preferably about 20-fold or less, for instance, even about 18-fold or less. The minimum degree of swelling is theoretically 1-fold; it can be about 3-fold or greater, for instance, about 5-fold or greater. The degree of swelling can be adjusted through, for instance, the base polymer's composition and its content (percentage), the base polymer's polymerization method and conditions, the base polymer's molecular weight, the type pf crosslinking agent (interfunctional-group distance) and its amount used, species of additives, etc. The degree of swelling is determined by the method described below.

Thickness of PSA Layer

No particular limitations are imposed on the thickness of the PSA layer constituting the PSA sheet disclosed herein. From the standpoint of preventing adhesive transfer to the adherend, the thickness of the PSA layer is about 50 μm or less, suitably about 30 μm or less, preferably about 15 μm or less, or more preferably about 8 μm or less (e.g. less than 6 μm). In other embodiments, from the standpoint of the ease of removal, etc., the thickness of the PSA layer is suitably about 5 μm or less, or possibly about 4 μm or less, for instance, 3 μm or less. From the standpoint of the adhesion, the thickness of the PSA layer is suitably about 0.5 μm or greater, preferably about 1 μm or greater, or more preferably greater than 2 μm. The thickness of the PSA layer may be greater than 3 μm, for instance, greater than 4 μm.

Substrate Layer

The PSA sheet disclosed herein may include a substrate layer. As the substrate layer, resin film, a rubber sheet, a foam sheet, a composite of these, etc., can be used. Examples of the rubber sheet include natural rubber sheets and butyl rubber sheets. Examples of the foam sheet include polyurethane foam sheets, and polychloroprene rubber foam sheets.

The art disclosed herein can be preferably applied to a PSA sheet wherein the substrate layer is resin film. The concept of “resin film” here refers to film typically obtained by molding a thin layer from a resin composition primarily comprising resin components as described below; it should be distinguished from so-called non-woven and woven fabrics. In other words, the concept of resin film excludes non-woven and woven fabrics. Preferable resin film is essentially not foamed. In other words, non-foamed resin film can be preferably used. Here, the non-foamed resin film refers to resin film that has not been deliberately subjected to a foaming process. In particular, the resin film may have an expansion ratio lower than about 1.1 (e.g. lower than 1.05, typically lower than 1.01).

Examples of the resin components forming the resin film include polyolefmic resins (polyethylene, polypropylene, ethylene-propylene copolymer, ethylene-vinyl acetate copolymer, etc.), poly(vinyl chloride)-based resins (typically soft poly(vinyl chloride)-based resin); poly(vinyl acetate)-based resin, polyurethane-based resins (ether-based polyurethane, ester-based polyurethane, carbonate-based polyurethane, etc.), urethane (meth)acrylate-based resin, thermoplastic elastomers (olefinic elastomer, styrene-based elastomer, acrylic elastomer, etc.), polyesterbased resins (polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, polybutylene naphthalate, etc.), polycarbonate-based resin, polyamide-based resin, polyimide-based resin, fluororesin and cellulose-based resins such as cellophane resin. Among these resins, solely one species or a combination of two or more species can be used. Favorable examples of the resin film include polyolefinic resin film, polyester-based resin film and vinyl chloride-based resin film.

While no particular limitations are imposed, in the PSA sheet according to an embodiment, it is preferable to use a substrate layer that comprises, as its primary component(s), one, two or more species of resin selected from the group consisting of polyolefinic resin, poly(vinyl chloride)-based resin, polyurethane-based resin, thermoplastic elastomer and polyesterbased resin (typically a substrate layer comprising such resin in an amount exceeding 50% by weight). In other embodiments, in view of the performance, ease of handling, costs, etc., a substrate layer comprising a polyolefinic resin layer, polyester-based resin layer or polyvinyl chloride-based resin layer can be preferably used. Among the resin materials, in view of the heat stability, the lightness of weight, etc., polyolefinic resins, polyurethane-based resins and olefinic elastomers are preferable; in view of the handling properties, etc., polyolefinic resins and olefinic elastomers are particularly preferable.

The PSA sheet disclosed herein can be preferably made in an embodiment comprising a substrate layer that comprises a polyolefinic resin as the primary component (e.g. more than 50% by weight), that is, an embodiment wherein the substrate layer is polyolefinic resin film. For instance, it is preferable to use polyolefinic resin film in which 50% by weight or more of the entire substrate layer is polyethylene (PE) resin or polypropylene (PP) resin. In other words, in the polyolefinic resin film, the combined amount of PE resin and PP resin may account for 50% by weight or more of the entire substrate layer. The polyolefinic resin film can be a blend of PE and PP resins.

The PP resin may comprise, as the primary component, various polymer species (propylene-based polymers) that comprise propylene as a monomer unit. The PP resin may be essentially formed of one, two or more species of propylene-based polymer. The concept of propylene-based polymer here includes homopolypropylene as well as a random copolymer of propylene and other monomer(s) (random polypropylene) and a block copolymer (block polypropylene). The concept of propylene-based polymer here includes, for instance, the following species:

Propylene homopolymer (homopolypropylene), for instance, isotactic polypropylene;

Random copolymer (random polypropylene) of propylene and other α-olefin(s) (typically, one, two or more species selected from ethylene and α-olefins having 4 to 10 carbon atoms), preferably random polypropylene comprising propylene as the primary monomer (i.e. the monomer accounting for 50% by weight or more of the total monomer content);

Block copolymer (block polypropylene) of propylene and other α-olefin(s) (typically, one, two or more species selected from ethylene and α-olefins having 4 to 10 carbon atoms), preferably block polypropylene comprising propylene as the primary monomer (i.e. the monomer accounting for 50% by weight or more of the total monomer content).

The PE resin may comprise, as the primary component, various types of polymer (ethylene-based polymer) comprising ethylene as a monomer. The PE resin may be essentially formed of one, two or more species of ethylene-based polymer. The ethylene-based polymer can be an ethylene homopolymer or a copolymer (random copolymer, block copolymer, etc.) of ethylene as the primary monomer and other α-olefin(s) as secondary monomer(s). Favorable examples of the a-olefins include α-olefins having 3 to 10 carbon atoms such as propylene, 1-butene (which can be a branched 1-butene), 1-hexene, 4-methyl-1-pentene and Foctene. For instance, it is preferable to use PE resin that comprises, as the primary component, an ethylene-based polymer in which an α-olefin as the secondary monomer is copolymerized up to about 10% by weight (typically up to about 5% by weight).

The PE resin may comprise a copolymer of ethylene and a monomer (functional monomer) containing other functional group(s) in addition to a polymerizable functional group, copolymer of an ethylene-based polymer copolymerized with such a functional monomer, or the like. Examples of a copolymer of ethylene and a functional monomer include ethylene-vinyl acetate copolymers (EVA), ethylene-acrylic acid copolymers (EAA), ethylene-methacrylic acid copolymers (EMAA), ethylene-methyl acrylate copolymers (EMA), ethylene-ethyl acrylate copolymers (EEA), ethylene-methyl methacrylate copolymers (EMMA), and copolymers of ethylene and (meth)acrylic acid (i.e. acrylic acid and/or methacrylic acid) crosslinked by metal ions.

The PE resin is not particularly limited in density. The concept of PE resin here includes all of the following: high density polyethylene (HDPE), medium density polyethylene (MDPE), low density polyethylene (LDPE) and linear low density polyethylene (LLPDE). In an embodiment, the density of the PE resin can be, for instance, about 0.90 g/cm³ to 0.94 g/cm³. Preferable PE resins include LDPE and LLDPE. The PE resin may comprise one, two or more species of LDPE and one, two or more species of LLDPE. There are no particular limitations to the respective blend ratios of LDPE and LLDPE, or to the LDPE to LLDPE blend ratio. They can be suitably selected to form a PE resin having desirable properties. As the substrate layer of the PSA sheet disclosed herein, it is preferable to use polyethylenic resin film such as LLDPE film whose LLDPE content is higher than 50% by weight (preferably about 75% by weight or higher, e.g. about 90% by weight or higher) and LDPE film whose LDPE content is higher than 50% by weight (preferably about 75% by weight or higher, e.g. about 90% by weight or higher). Laminate resin film comprising such polyethylenic resin film as a component can be used as well.

The resin film (e.g. polyolefinic resin film) used as the substrate layer of the PSA sheet disclosed herein may comprise, as necessary, suitable components allowable in the substrate layer. Examples of additives that can be suitably added include filler, colorant (pigment such as inorganic pigment, dye), antioxidant, photostabilizer (including radical scavenger and UV absorber), antistatic agent, plasticizer, slip agent, and anti-blocking agent. Each additive can be added, for instance, in an amount similar to a typical amount in the field of resin film used as substrate layers and the like of PSA sheets.

The substrate layer may have a mono-layer structure or a multi-layer structure formed of two, three or more layers. In a multi-layer structure, it is preferable that at least one layer (preferably each layer) is formed of aforementioned resin film. For instance, in a preferable substrate layer, 75% or more (more preferably 90% or more) of the thickness is attributed to mono-layer or multi-layer (typically mono-layer) polyolefinic resin film. The substrate layer may be entirely formed of mono-layer or multi-layer polyolefinic resin film. From the standpoint of the cost-effectiveness, it is preferable to use a substrate layer formed of mono-layer resin film (e.g. LLDPE film, LDPE film, etc.).

The method for producing the substrate layer can be suitably selected among heretofore known methods and is not particularly limited. For instance, when resin film is used as the substrate layer, it is possible to use resin film fabricated by suitably employing a heretofore known general film-forming method such as inflation molding, extrusion, T-die cast molding, and calendar roll molding.

In an embodiment where at least one face (the PSA layerside face) of the substrate layer is a resin film surface, the resin film surface can be subjected to a heretofore known surface treatment such as corona discharge treatment, plasma treatment, ozone exposure, flame exposure, UV irradiation, acid treatment, alkali treatment, and primer coating. These surface treatments may enhance the tightness of adhesion between the substrate layer and the PSA layer, or the anchoring of the PSA layer onto the substrate layer. In an embodiment using polyolefinic resin film as the substrate layer, it is particularly meaningful to provide these surface treatments.

The thickness of the substrate layer constituting the PSA sheet disclosed herein is not particularly limited. The thickness of the substrate layer can be, for instance, about 800 μm or less (typically about 250 μm or less). In some embodiments, the thickness of the substrate layer (typically, non-foamed resin film) is preferably about 150 μm or less, more preferably about 100 μm or less, or yet more preferably less than 65 μm, for instance, less than 55 μm. With decreasing thickness of the substrate layer, the PSA sheet tends to exhibit greater conformability to the adherend shape while its lifting and peeling tend to be inhibited. From the standpoint of adherend protection and handling properties, etc., the thickness of the substrate layer can be typically about 10 μm or greater, preferably about 25 μm or greater, more preferably greater than about 30 μm, or yet more preferably greater than 40 μm, or possibly, for instance, greater than 45 μm.

Method for Preparing PSA Composition

The method for preparing the PSA composition disclosed herein is not particularly limited and a heretofore known or conventional PSA composition preparation method can be suitably employed. While no particular limitations are imposed, the PSA composition preparation method according to some embodiments preferably uses an aqueous liquid that satisfies (a) having an electrical conductivity below 300 μS/cm, (b) having a chloride ion concentration below 35 μg/mL or both. According to the method, it is possible to obtain a PSA composition that can prevent metal corrosion. Such a PSA composition is particularly preferably used in applications adhered to metal plates.

Obtaining of Aqueous Liquid

The method for preparing the PSA composition according to some embodiments may include a step of obtaining the aqueous liquid. The aqueous liquid satisfying the (a) and/or (b) can be obtained by selecting, upon analysis if necessary, an aqueous liquid that has a desirable electrical conductivity and/or a chloride ion concentration among raw water including surface water, groundwater (possibly well water), tap water and treated water obtained upon known water treatment.

In some preferable embodiments, the aqueous liquid is obtained by subjecting an aqueous liquid (untreated aqueous liquid) such as raw water to a known treatment such as filtration, membrane separation, ion exchange, distillation, physical adsorption, and electrolysis. In particular, a more preferable treatment is membrane separation using a reverse osmosis membrane, nanofiltration membrane, ultrafiltration membrane, precision filtration membrane and the like with the reverse osmosis membrane being particularly preferable. When ion exchange is selected as the water treatment method, cation exchange (using cation exchange resin), anion exchange (using anion exchange resin) or a mixed bed thereof can be used; however, from the standpoint of removing anions (typically chloride ions) as primary contributors to metal corrosion, anion exchange is preferably selected when using ion-exchanged water. In this embodiment, the untreated aqueous liquid used in the treatment process has an electrical conductivity of 300 μS/cm or higher and a chloride ion concentration of 35 μg/mL or greater. The treatment system is not particularly limited and a known or commonly-used treatment system can be used. The treatment conditions can be suitably set in accordance with the untreated water, system and required water quality without particular limitations.

Preparation of PSA Composition

Subsequently, the aqueous liquid obtained as described above is used to obtain a polymer-containing liquid that comprises a polymer (base polymer) as an adhesive component. In other words, the method disclosed herein may include a step of obtaining a polymer-containing liquid comprising the aqueous liquid and a polymer. The usage of the aqueous liquid is not particularly limited as long as it is used for preparing a PSA composition. The timing for using it is not particularly limited, either. The aqueous liquid is also referred to as water for PSA composition preparation (or PSA composition prep water). The prep water can be water used in the polymerization (hereinafter as “polymerization water”), water added after the polymerization (water added afterwards), etc. The resulting polymer-containing liquid can be used as the PSA composition or it can be further mixed with additives to prepare the PSA composition. The PSA composition obtained using the aqueous liquid can be a water-dispersed PSA composition in which the polymer is at least partially (typically entirely) dispersed in the aqueous liquid. The particulars of the polymer (typically the base polymer) used in the PSA composition as well as the particulars of the PSA composition and other matters related to preparation of the PSA composition are as described earlier and redundant details are thus omitted.

In some preferable embodiments, the aqueous liquid is used as polymerization water in emulsion polymerization. In other words, the PSA composition preparation method disclosed herein may include a step of obtaining a polymer by carrying out emulsion polymerization in the presence of the aqueous liquid. The particulars of the polymer obtained by emulsion polymerization are as described earlier and redundant details are thus omitted.

Properties of Aqueous Liquid

In some embodiments, the aqueous liquid used for preparing the PSA composition is not particularly limited in electrical conductivity. Chloride ions in the aqueous liquid are preferably limited to below the prescribed quantity. In such a case, it is not essential to have an electrical conductivity below 300 μS/cm. On the other hand, because the electrical conductivity correlates with the ion amount in the aqueous liquid, from the standpoint of limiting the metal-corrosive ion amount, the electrical conductivity of the aqueous liquid in other embodiments is preferably below 300 μS/cm, more preferably below 200 μS/cm, yet more preferably below 100 μS/cm, or particularly preferably below 50 μS/cm (e.g. below 30 μS/cm, or even below 15 μS/cm). Such an aqueous liquid can be used as an anti-metal-corrosive medium without quantifying chloride ions which are considered a primary contributor to metal corrosion.

The minimum electrical conductivity of the aqueous liquid used can be the theoretical lower limit (around 0.05 μS/cm at room temperature). In view of the productivity, allowable level for practical use, etc., it can be about 1 μS/cm or higher, about 5 μS/cm or higher, about 10 μS/cm or higher, about 50 μS/cm or higher (e.g. about 100 μS/cm or higher, or even about 200 μS/cm or higher).

The electrical conductivity of the aqueous liquid is determined at room temperature (23° C.), using a commercial electrical conductivity (EC) meter. In particular, a portable EC meter ES-71 available from Horiba, Ltd. or a comparable product can be used for the measurement.

In some embodiments, the aqueous liquid used for preparing the PSA composition is preferably limited in chloride ion concentration. In particular, when the aqueous liquid has an electrical conductivity of 300 μS/cm or higher, it preferably has a chloride ion concentration below 35 μg/mL. This can bring about anti-metal-corrosion effects. While no particular limitations are imposed, the chloride ion concentration in the aqueous liquid is preferably below 35 μg/mL even when the electrical conductivity is below 300 μS/cm. In this case, the aqueous liquid used in the preparation of PSA composition disclosed herein comprises less than 35 μg/mL of chloride ions regardless of electrical conductivity.

In some embodiments, the chloride ion concentration in the aqueous liquid is suitably below 35 μg/mL (e.g. below 30 μg/mL), preferably below 25 μg/mL, more preferably below 20 μg/mL, yet more preferably below 15 μg/mL, particularly preferably below 10 μg/mL, most preferably below 5 μg/mL, or possibly even below 3 μg/mL (e.g. below 1 μg/mL). The minimum chloride ion concentration in the aqueous liquid is ideally 0 μg/mL; however, in view of the productivity, allowable level for practical use, etc., it can be about 1 μg/mL or greater, or even about 3 μg/mL or greater.

In some preferable embodiments, the aqueous liquid has a ratio (A_(C1)/C) of chloride ion concentration A_(C1) (μg/mL) to electrical conductivity C (μS/cm) of about 1/5 or lower. That the A_(C1)/C ratio is at or below the prescribed value means that the chloride ion concentration is relatively low among all ions. When the A_(C1)/C ratio is about 1/5 or lower, prevention of metal corrosion can be preferably combined with productivity. The A_(C1)/C ratio is more preferably about 1/8 or lower, about 1/10 or lower, or about 1/15 or lower. The minimum A_(C1)/C ratio is not particularly limited and can be about 1/30 or higher (e.g. 1/20 or higher).

From the standpoint of preventing metal corrosion, the aqueous liquid used in the preparation of PSA composition according to some embodiments is desirably limited in sulfate ion concentration. The sulfate ion concentration in the aqueous liquid is about 50 μg/mL or less, suitably about 35 μg/mL or less (e.g. about 25 μg/mL or less), preferably about 15 μg/mL or less, more preferably about 5 μg/mL or less, or yet more preferably about 2 μg/mL or less (e.g. about 1 μg/mL or less). The minimum sulfate ion concentration in the aqueous liquid is ideally 0 μg/mL; however, in view of the productivity, allowable level for practical use, etc., it can be about 10 μg/mL or greater, or even about 20 μg/mL or greater.

From the standpoint of preventing metal corrosion, the aqueous liquid used in the preparation of PSA composition according to some embodiments is desirably limited in nitrate ion concentration. The nitrate ion concentration in the aqueous liquid is about 30 μg/mL or less, suitably about 20 μg/mL or less (e.g. about 10 μg/mL or less), preferably about 7 μg/mL or less, more preferably about 5 μg/mL or less, or yet more preferably about 2 μg/mL or less (e.g. about 1 μg/mL or less). The minimum nitrate ion concentration in the aqueous liquid is ideally 0 μg/mL; however, in view of the productivity, allowable level for practical use, etc., it can be about 1 μg/mL or greater, about 3 μg/mL or greater, or even about 5 μg/mL or greater (e.g. 10 μg/mL or greater).

In the aqueous liquid used for preparing the PSA composition according to some embodiments, the total concentration of nitrate, sulfate and chloride ions is suitably about 80 μg/mL or less. By limiting the total concentration of these metal-corrosive ion species, metal corrosion can be more assuredly prevented. The total concentration of the three ion species is preferably below 50 μg/mL, more preferably below 30 μg/mL, yet more preferably below 15 μg/mL, or possibly about 10 μg/mL or less (e.g. about 5 μg/mL or less). The minimum total concentration of the three ion species in the aqueous liquid is ideally 0 μg/mL; however, from the standpoint of the productivity, allowable level for practical use, etc., it can be about 1 μg/mL or greater, about 5 μg/mL or greater, or even about 10 μg/mL or greater (e.g. 30 μg/mL or greater).

From the standpoint of preventing metal corrosion, in the art disclosed herein, among the ions in the aqueous liquid, it is preferable to limit the anion concentration (typically the chloride ion concentration). On the other hand, the cation concentration in the aqueous liquid is not particularly limited. In some embodiments, the cation concentration in the aqueous liquid can be in the range where the electrical conductivity is below 300 μS/cm. For instance, the concentration of calcium and magnesium ions removed by a general water-softening process is not particularly limited. The concentration of calcium and magnesium ions in the aqueous liquid used in the preparation of PSA composition can be 1 μg/mL or greater, about 10 μg/mL or greater, or even 20 μg/mL or greater.

The concentrations of chloride ions, sulfate ions, nitrate ions, calcium ions and magnesium ions in the aqueous liquid can be determined by ion chromatography using the respective commercial standard ion solutions (e.g. available from Wako Pure Chemical industries, Ltd.). In particular, they are determined by the method described below.

Quantification of Ions in Aqueous Liquid

Ion chromatography analysis was carried out, using the respective commercial standard ion solutions (available from Wako Pure Chemical industries, Ltd.). The ion chromatography conditions were as shown below.

Analytical Conditions

Anion analysis:

System: ICS-3000 available from Thermo Fisher Scientific

Separation column: Dionex IonPac AS18-Fast (4 mm×250 mm)

Guard column: Dionex IonPac AG18-Fast (4 mm×50 mm)

Removal system: Dionex AERS-500 (external mode)

Detector: electrical conductivity detector

Eluent: aqueous KOH solution (eluent generator cartridge used)

Eluent flow rate: 1.0 mL/min

Sample injection volution: 250 μL

Cation analysis:

System: DX-320 available from Thermo Fisher Scientific

Separation column: Dionex Ion Pac CS16 (5 mm×250 mm)

Guard column: Dionex Ion Pac CG16 (5 mm×50 mm)

Removal system: Dionex CSRS-500 (recycle mode)

Detector: electrical conductivity detector

Eluent: aqueous methanesulfonic acid solution

Eluent flow rate: 1.0 ml/min

Sample injection volution: 25 μL

The aqueous liquid disclosed herein refers to a liquid whose primary component is water (H₂O) and it may include an organic solvent such as alcohol at a lower percentage than that of water. In typical, a liquid comprising more than 50% water by volume is used as the aqueous liquid. The water content of the aqueous liquid is, for instance, about 90% by volume or higher, or suitably about 99% by volume or higher (e.g. 99.9% by volume or higher).

Method for producing PSA Sheet

The art disclosed herein includes a method for producing a PSA sheet. The PSA sheet production method may comprise a step of obtaining a PSA composition by the preparation method described above and a step of forming a PSA layer from the resulting PSA composition. Because PSA-caused metal corrosion can be prevented, the PSA sheet having such a PSA layer is particularly preferably used in applications adhered to metal plates. When the PSA sheet has a substrate layer and a PSA layer, the method for producing the PSA sheet may include a step of providing a PSA layer to one face of the substrate layer.

The PSA layer can be formed based on a known method for forming PSA layers in PSA sheets. For instance, it is preferable to employ a direct method where the PSA composition is directly provided (typically applied) to the substrate and allowed to cure to form a PSA layer. Alternatively, it is also possible to employ a transfer method where the PSA composition is provided to a releasable surface (release face) and allowed to cure to form a PSA layer on the surface and the resulting PSA layer is transferred to the substrate. As the release face, the surface of a release liner, the substrate's backside treated with release agent, or the like can be used. While the PSA layer is typically formed in a continuous form, it can be formed in a regular or random pattern of dots, stripes, etc.

The PSA composition can be applied, using a heretofore known coater, for instance, a gravure roll coater, reverse roll coater, kiss roll coater, dip roll coater, die coater, bar coater, knife coater and spray coater. Alternatively, the PSA composition can be applied by immersion, curtain coating, etc. From the standpoint of accelerating the crosslinking reaction, increasing the productivity, etc., the PSA composition is preferably dried with heat. The drying temperature is possibly, for example, 40° C. to 150° C. or preferably 50° C. to 130° C. (e.g. 60° C. to 100° C.). After dried, the PSA composition can be allowed to further age for purposes such as adjusting the distribution or migration of components in the PSA layer, advancing the crosslinking reaction, and lessening the strain possibly present in the substrate and the PSA layer.

Applications

The PSA sheet disclosed herein is not particularly limited in application and can be applied to a PSA sheet used for various purposes. The PSA sheet disclosed herein shows excellent aged removability from adherend and thus preferably used as a surface protective sheet which is applied to the surfaces of metal plates, coated steel plates, synthetic resin plates, glass plates and the like to prevent damage (scratches, contamination, etc.) caused to these surfaces; and is removed from the adherend after completion of the protection period. Such a surface protective sheet is formed of an adhesively single-faced PSA sheet typically having a PSA layer on one face of a substrate. The PSA sheet disclosed herein has anti-metal-corrosive properties; and therefore, in some preferable embodiments, it can be preferably used for protecting a metal plate formed from a metal such as stainless-steel, aluminum, iron, etc. The metal plate (e.g. an aluminum plate) may be subjected to the sort of surface coating after the protection period by the PSA sheet ends. Thus, it is important to highly inhibit adherend contamination. By applying the art disclosed herein for metal plate protection, after the protection by the PSA sheet ends, the metal plate may still be free of defects (coating defects, etc.) even in post-processes such as coating.

The thickness of the metal plate to be protected by the PSA sheet is not particularly limited and can be, for instance, about 40 mm or less. In some embodiments, the metal plate's thickness is about 30 mm or less, or possibly about 20 mm or less. The minimum thickness of the metal plate is, for instance, about 0.2 mm or greater, possibly about 0.3 mm or greater, or even about 0.6 mm or greater. The PSA sheet disclosed herein can exhibit good aged removability to a metal plate having a thickness in these ranges.

The PSA sheet disclosed herein may show suppressed aged adhesive strength and thus may provide good efficiency of removal even when, for instance, the period of adhesion to the adherend (which can be the protection period for the adherend) becomes relatively long (typically two weeks or longer, e.g. four weeks or longer). Thus, for instance, it can be favorably used in an embodiment where the PSA sheet is removed from the adherend (in particular, a metal plate) after being applied thereon for two weeks or more (e.g. four weeks or more).

Several working examples related to the present invention are described below, but the present invention should not be limited to these examples. In the description below, “part(s)” and “%” are by weight unless otherwise specified.

Test Methods Determination of Gel Fraction and Degree of Swelling

A PSA layer sample (weight: W₁) weighing approximately 0.1 g is wrapped into a pouch with a porous polytetrafluoroethylene membrane (weight: W₂) having a mean pore diameter of 0.2 μm, and the opening is tied with twine (weight: W₃). As the porous polytetrafluoroethylene membrane, product name NITOFLON® NTF 1122 (available from Nitto Denko Corporation; 0.2 μm mean pore diameter, 75% porosity, 85 μm thickness) or an equivalent product is used. The resulting package is immersed in 50 mL of ethyl acetate and stored at room temperature (typically 23° C.) for 7 days. Subsequently, the package is taken out, and any residual ethyl acetate is wiped off the outer surface. The package weight (W₄) is measured. The package is then dried at 130° C. for 2 hours and the package weight (W₅) is measured. The gel fraction and the degree of swelling of the PSA layer can be determined by substituting the respective values into the following equations:

Gel  fraction  (%) = [(W₅ − W₂ − W₃)/W₁] × 100 Degree  of  swelling  (fold) = (W₄ − W₂ − W₃)/(W₅ − W₂ − W₃)

Initial Peel Strength (Initial to-SUS Adhesive Strength)

The PSA sheet to be measured is cut to a 20 mm wide by 100 mm long strip to prepare a test piece. In a standard environment at 23° C., 50% RH, with a 2 kg rubber roller moved back and forth twice, the test piece is press-bonded to a stainless-steel plate (SUS430BA plate) as the adherend. The sample is stored in the standard environment for 30 minutes. Subsequently, in the same standard environment, using a universal tensile tester, the initial peel strength (initial to-SUS adhesive strength) (N/20 mm) is determined at a tensile speed of 0.3 m/min, at 180° peel angle. In case of an adhesively double-faced PSA sheet, it is backed with about 25μm thick PET film and subjected to measurement.

Initial Peel Strength (Initial to-Aluminum Adhesive Strength)

The PSA sheet to be measured is cut to a 20 mm wide by 100 mm long strip to prepare a test piece. In a standard environment at 23° C., 50% RH, with a 2 kg rubber roller moved back and forth twice, the test piece is press-bonded to an aluminum plate (AL5052 plate) as the adherend. The sample is stored in the standard environment for 30 minutes. Subsequently, in the same standard environment, using a universal tensile tester, the initial peel strength (initial to-aluminum adhesive strength) (N/20 mm) is determined at a tensile speed of 0.3 m/min, at 180° peel angle. In case of an adhesively double-faced PSA sheet, it is backed with about 25 μm thick PET film and subjected to measurement.

Aged Peel Strength (Aged to-SUS Adhesive Strength)

The PSA sheet to be measured is cut to a 20 mm wide by 100 mm long strip to prepare a test piece. In a standard environment at 23° C., 50% RH, with a 2 kg rubber roller moved back and forth twice, the test piece is press-bonded to a stainless-steel plate (SUS430BA plate) as the adherend. The sample is stored in an environment at 50° C. for seven days and then in a standard environment at 23° C., 50% RH for one hour. Subsequently, in the same standard environment, using a universal tensile tester, the aged peel strength (aged to-SUS adhesive strength) (N/20 mm) is determined at a tensile speed of 0.3 m/min, at 180° peel angle. In case of an adhesively double-faced PSA sheet, it is backed with about 25 μm thick PET film and subjected to measurement.

Aged Peel Strength (Aged to-Aluminum Adhesive Strength)

The PSA sheet to be measured is cut to a 20 mm wide by 100 mm long strip to prepare a test piece. In a standard environment at 23° C., 50% RH, with a 2 kg rubber roller moved back and forth twice, the test piece is press-bonded to an aluminum plate (AL5052 plate) as the adherend. The sample is stored in an environment at 50° C. for seven days and then in a standard environment at 23° C., 50% RH for one hour. Subsequently, in the same standard environment, using a universal tensile tester, the aged peel strength (aged to-aluminum adhesive strength) (N/20 mm) is determined at a tensile speed of 0.3 m/min, at 180° peel angle. In case of an adhesively double-faced PSA sheet, it is backed with about 25 μm thick PET film and subjected to measurement.

Chloride Ion Amount of PSA (Layer)

An approximately 10 mg of the adhesive components (PSA) of the PSA sheet to be measured is placed in a polypropylene (PP) container and weighed. To this, 100 mL of super pure water is added, lidded, and subjected to thermal extraction for one hour in a dryer at 120° C. With respect to the resulting extract, by ion chromatography, chloride ions in the extract are quantified using commercial standard chloride ion solutions (available from FUJIFILM Wako Pure Chemical industries, Ltd.) to determine the chloride ion amount per gram of PSA sheet used in the extraction. The analytical conditions for ion chromatography are the same as the measurement conditions for the chloride ion amount of PSA.

Analytical Conditions

Anion analysis:

System: ICS-3000 available from Thermo Fisher Scientific

Separation column: Dionex IonPac AS18-Fast mm×250 mm)

Guard column: Dionex IonPac AG18-Fast (4 mm×50 mm)

Removal system: Dionex AERS-500 (external mode)

Detector: electrical conductivity detector

Eluent: aqueous KOH solution (eluent generator cartridge used)

Eluent flow rate: 1.0 mL/min

Sample injection volution: 250 μL

Cation analysis:

System: DX-320 available from Thermo Fisher Scientific

Separation column: Dionex Ion Pac CS16 (5 mm×250 mm)

Guard column: Dionex Ion Pac CG16 (5 mm×50 mm)

Removal system: Dionex CSRS-500 (recycle mode)

Detector: electrical conductivity detector

Eluent: aqueous methanesulfonic acid solution

Eluent flow rate: 1.0 mL/min

Sample injection volution: 25 μL

Chloride Ion Amount of PSA Sheet

The PSA sheet subject to measurement is washed well with pure water, cut to a 50 cm² sized sheet area, placed in a polypropylene (PP) container and weighed. Using a drier, thermal extraction is carried out at 120° C. for one hour. 25 mL of pure water is added to the PP container. Using a drier, thermal extraction is carried out at 120° C. for one hour. With respect to the resulting extract, by ion chromatography, chloride ions in the extract are quantified using commercial standard chloride ion solutions (available from Wako Pure Chemical industries, Ltd.) to determine the chloride ion amount per gram of PSA sheet used in the extraction. The analytical conditions for ion chromatography are the same as the measurement conditions for the chloride ion amount of PSA.

Removability of PSA Sheet

When a thin metal plate is used as the adherend, an aged peel strength above 2.7 N/20 mm tends to result in poor removability with a bend in the adherend, etc. Based on this knowledge, it is graded as “Good” when the aged to-SUS adhesive strength and aged to-aluminum adhesive strength were at or below 2.7 N/20 mm, and as “Poor” when above 2.7 N/20 mm.

Corrosion Test of Aluminum Plate

The PSA sheet is cut to a 20 mm wide, 100 mm long strip to prepare a test piece. In a standard environment at 23° C. and 50% RH, the test piece is press-bonded to an aluminum plate (AL5052 plate) as the adherend with a 2 kg rubber roller moved back and forth twice. The sample is stored in an environment at 40° C. and 92% RH for 7 days and the PSA sheet is then removed. With respect to the area where the PSA sheet had been adhered, the state of the aluminum plate surface is visually inspected. The following grades are assigned: “Excellent” when no discoloration is observed, “Good” when a practically acceptable level of minor discoloration is observed, and “Fail” when discoloration is observed.

EXAMPLE 1

Were mixed a starting monomer mixture comprising 58 parts of 2-ethylhexyl acrylate, 40 parts of n-butyl methacrylate and 2 parts of acrylic acid with 3 parts of surfactant (trade name AQUALON KH-1025 available from Dai-ichi Kogyo Seiyaku Co., Ltd.; ammonium polyoxyethylene-1-(allyloxymethyl) alkyl ether sulfate with up to 30 moles of added EO) and 150 parts of polymerization water. The resulting mixture was emulsified under nitrogen flow with an emulsifying machine (homo mixer) to prepare an emulsion of the starting monomers.

Into a reaction vessel equipped with a thermometer, nitrogen inlet, condenser and stirrer, was placed the emulsion and heated under nitrogen flow to a liquid temperature of 50° C. while stirring. To this, was added 0.03 part of 2,2′-azobis(2-methylpropionamidine)dihydrochloride (product name V-50 available from Wako Pure Chemical industries, Ltd.) as polymerization initiator and the polymerization reaction was carried out for 5 hours while maintaining the liquid temperature around 50° C. To the resulting polymerization reaction mixture, was added ammonia water and adjusted to around pH 8. An aqueous dispersion of acrylic polymer was thus prepared.

To the water dispersion, per 100 parts of non-volatiles therein, were mixed 2 parts of oxazoline-based crosslinking agent (product name EPOCROS WS-500 available from Nippon Shokubai Co., Ltd.) and 0.1 part of rust inhibitor A (product name LAMIPROOF C-2 available from DKS Co., Ltd.; polyoxyethylene-containing alicyclic amine) to prepare a PSA composition according to this Example.

Of 55 μm thick polyethylene film having one face treated with corona discharge, onto the corona-treated face (first face), was applied the PSA composition with a bar coater to a dried thickness of 6 μm. This was dried at 70° C. for two minutes and then aged at 50° C. for one day to prepare an adhesively single-faced PSA sheet having an acrylic PSA layer on one face of a substrate layer formed of the polyethylene film. A PSA sheet according to this Example was thus obtained.

EXAMPLES 2 to 5

Rust inhibitor A was used in the amounts show in Table 1. Otherwise in the same manner as Example 1, were obtained PSA sheets according to the respective Examples.

EXAMPLES 6 to 7

Rust inhibitor A was changed to rust inhibitor B (product name CHELESLITE W-16B available from Chelest Corporation, containing organic acid salt and rust inhibitor for iron) or to rust inhibitor C (product name THIOLITE C-560R13 available from Chiyoda Chemical Co., Ltd., 50-60% triethanolamine). Otherwise in the same manner as Example 3, were obtained PSA sheets according to the respective Examples.

EXAMPLE 8

To the PSA composition according to Example 3, was further added aqueous 1% NaCl solution to obtain a PSA composition according to this Example. Using this PSA composition, but otherwise in the same manner as Example 3, was obtained a PSA sheet according to this Example.

With respect to the PSA sheet of each Example, Table 1 shows the summarized features, gel fraction (%), degree of swelling (fold), initial to-SUS adhesive strength (N/20 mm), initial to-aluminum adhesive strength (N/20 mm), aged to-SUS adhesive strength (N/20 mm), aged to-aluminum adhesive strength (N/20 mm), chloride ion amount per gram of PSA by hot water extraction (μg/g(PSA)), chloride ion amount per gram of PSA sheet by hot water extraction (μg/g(PSA sheet)), results of PSA sheet's removability and aluminum corrosion test.

TABLE 1 Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 7 Ex. 8 PSA Base polymer Acrylic polymer (pts) 100 100 100 100 100 100 100 100 Reactive surfactant (pts) 3 3 3 3 3 3 3 3 Crosslinking agent (pts) 2 2 2 2 2 2 2 2 Additive Rust inhibitor A (pts) 0.1 0.5 1.4 2.8 8.3 — — 1.4 Rust inhibitor B (pts) — — — — — 1.4 — — Rust inhibitor C (pts) — — — — — — 1.4 — Percentage of base polymer 99.9% 99.5% 98.7% 97.4% 92.7% 98.7% 98.7% 98.7% Gel fraction   87%   87%   77%   73%   60%   87%   85%   77% Degree of swelling (fold) 17 18 17 19 24 17 17 17 Chloride ion amount PSA (μg/g) 74 71 102 132 215 71 64 356 Protective sheet (μg/g) 7.3 7.0 10.0 13.0 21.1 7.0 6.3 35.0 To-SUS adheisve strength Initial (N/20 mm) 1.8 1.8 1.7 2.2 2.3 1.9 1.8 1.7 Aged (N/20 mm) 2.1 2.1 1.8 2.6 3.2 2.4 2.4 1.9 To-Al adhesive strength Initial (N/20 mm) 1.3 1.2 1.5 1.6 1.8 1.6 1.5 1.7 Aged (N/20 mm) 1.5 1.4 1.7 2.3 3.0 2.4 1.8 2.0 Evaluations PSA sheet's removability test Good Good Good Good Poor Good Good Good Aluminum corrosion test Good Good Good Excellent Excellent Good Good Fail

As shown in Table 1, the PSA sheets according to Examples 1 to 4 and 6 to 7 having PSA layers comprising at least 95% base polymer by weight were limited in both initial and aged adhesive strength, showing good removability. In these Examples, adherend surface contamination was not observed, either. In contrast, with respect to Example 5 comprising less than 95% base polymer by weight, the PSA sheet showed poor removability. The PSA sheets according to Examples 1 to 4 and 6 to 7 all had less than 300 μg of chloride ions per gram of PSA layer determined by hot water extraction and showed pass levels in the aluminum corrosion test as well. On the other hand, with respect to Example 8 having a chloride ion amount per gram of PSA layer of 300 μg or greater, corrosion was observed in the aluminum corrosion test. These results indicate that according to a PSA sheet that has a PSA layer comprising at least 95% base polymer by weight and has a chloride ion amount of less than 300 μg per gram of PSA layer determined by hot water extraction, because of its anti-metal-corrosive properties, it can also be used on a metal adherend; and even when prolongedly adhered to the adherend, it can maintain good removability while preventing or inhibiting contamination of the adherend surface.

Although specific embodiments of the present invention have been described in detail above, these are merely for illustrations and do not limit the scope of the claims. The art according to the claims includes various modifications and changes made to the

REFERENCE SIGNS LIST

-   1: substrate layer -   1A: first face -   1B: second face -   2: PSA layer -   2A: adhesive face -   10: PSA sheet 

1. A pressure-sensitive adhesive sheet having a pressure-sensitive adhesive layer, wherein the pressure-sensitive adhesive layer comprises at least 95% base polymer by weight, and has a chloride ion amount of less than 300 μg per gram of pressure-sensitive adhesive layer, determined by hot water extraction.
 2. The pressure-sensitive adhesive sheet according to claim 1, wherein the pressure-sensitive adhesive layer is formed from a water-dispersed pressure-sensitive adhesive composition.
 3. The pressure-sensitive adhesive sheet according to claim 1, wherein the pressure-sensitive adhesive layer comprises 3% rust inhibitor by weight or less.
 4. The pressure-sensitive adhesive sheet according to claim 3, wherein the rust inhibitor is an amine-based rust inhibitor.
 5. The pressure-sensitive adhesive sheet according to claim 1, wherein the base polymer is selected between an acrylic polymer and a rubber-based polymer.
 6. The pressure-sensitive adhesive sheet according to claim 1, wherein the base polymer is crosslinked with a crosslinking agent.
 7. The pressure-sensitive adhesive sheet according to claim 6, wherein the crosslinking agent is an oxazoline-based crosslinking agent.
 8. The pressure-sensitive adhesive sheet according to claim 1, having a substrate layer supporting the pressure-sensitive adhesive layer.
 9. The pressure-sensitive adhesive sheet according to claim 1, having a 180° peel strength of 2.7 N/20 mm or less to a metal plate selected between a stainless-steel plate and an aluminum plate after applied to the metal plate and stored at 50° C. for one week.
 10. The pressure-sensitive adhesive sheet according to claim 1, used for protecting a metal plate. 